Migration guide: pathprefixstrip migration

docs/content/migration/v1-to-v2.md

@@ -184,7 +184,7 @@ Then any router can refer to an instance of the wanted middleware.
               - "test2:$apr1$d9hr9HBB$4HxwgUir3HP4EsggP/QNo0"
     ```
 
-## TLS configuration is now dynamic, per router.
+## TLS Configuration Is Now Dynamic, per Router.
 
 TLS parameters used to be specified in the static configuration, as an entryPoint field.
 With Traefik v2, a new dynamic TLS section at the root contains all the desired TLS configurations.
@@ -322,7 +322,7 @@ Then, a [router's TLS field](../routing/routers/index.md#tls) can refer to one o
       - "traefik.http.routers.router0.tls.options=myTLSOptions@file"
     ```
 
-## HTTP to HTTPS Redirection is now configured on Routers
+## HTTP to HTTPS Redirection Is Now Configured on Routers
 
 Previously on Traefik v1, the redirection was applied on an entry point or on a frontend.
 With Traefik v2 it is applied on a [Router](../routing/routers/index.md). 
@@ -508,6 +508,137 @@ To apply a redirection, one of the redirect middlewares, [RedirectRegex](../midd
           keyFile: /app/certs/server/server.pem
     ```
 
+## Strip and Rewrite Path Prefixes
+
+With the new core notions of v2 (introduced earlier in the section
+["Frontends and Backends Are Dead... Long Live Routers, Middlewares, and Services"](#frontends-and-backends-are-dead-long-live-routers-middlewares-and-services)),
+transforming the URL path prefix of incoming requests is configured with [middlewares](../../middlewares/overview/),
+after the routing step with [router rule `PathPrefix`](https://docs.traefik.io/v2.0/routing/routers/#rule).
+
+Use Case: Incoming requests to `http://company.org/admin` are forwarded to the webapplication "admin",
+with the path `/admin` stripped, e.g. to `http://<IP>:<port>/`. In this case, you must:
+
+* First, configure a router named `admin` with a rule matching at least the path prefix with the `PathPrefix` keyword,
+* Then, define a middlware of type [`stripprefix`](../../middlewares/stripprefix/), which remove the prefix `/admin`, associated to the router `admin`.
+
+!!! example "Strip Path Prefix When Forwarding to Backend"
+
+    !!! info "v1"
+
+    ```yaml tab="Docker"
+    labels:
+      - "traefik.frontend.rule=Host:company.org;PathPrefixStrip:/admin"
+    ```
+
+    ```yaml tab="Kubernetes Ingress"
+    apiVersion: networking.k8s.io/v1beta1
+    kind: Ingress
+    metadata:
+      name: traefik
+      annotations:
+        kubernetes.io/ingress.class: traefik
+        traefik.ingress.kubernetes.io/rule-type: PathPrefixStrip
+    spec:
+      rules:
+      - host: company.org
+        http:
+          paths:
+          - path: /admin
+            backend:
+              serviceName: admin-svc
+              servicePort: admin
+    ```
+
+    ```toml tab="File (TOML)"
+    [frontends.admin]
+      [frontends.admin.routes.admin_1]
+      rule = "Host:company.org;PathPrefixStrip:/admin"
+    ```
+
+    !!! info "v2"
+
+    ```yaml tab="Docker"
+    labels:
+      - "traefik.http.routers.admin.rule=Host(`company.org`) && PathPrefix(`/admin`)"
+      - "traefik.http.middlewares.admin-stripprefix.stripprefix.prefixes=/admin"
+      - "traefik.http.routers.web.middlewares=admin-stripprefix@docker"
+    ```
+
+    ```yaml tab="Kubernetes IngressRoute"
+    ---
+    apiVersion: traefik.containo.us/v1alpha1
+    kind: IngressRoute
+    metadata:
+      name: http-redirect-ingressRoute
+      namespace: admin-web
+    spec:
+      entryPoints:
+        - web
+      routes:
+        - match: Host(`company.org`) && PathPrefix(`/admin`)
+          kind: Rule
+          services:
+            - name: admin-svc
+              port: admin
+          middlewares:
+            - name: admin-stripprefix
+    ---
+    kind: Middleware
+    metadata:
+      name: admin-stripprefix
+    spec:
+      stripPrefix:
+        prefixes:
+          - /admin
+    ```
+
+    ```toml tab="File (TOML)"
+    ## Dynamic configuration
+    # dynamic-conf.toml
+
+    [http.routers.router1]
+        rule = "Host(`company.org`) && PathPrefix(`/admin`)"
+        service = "admin-svc"
+        entrypoints = ["web"]
+        middlewares = ["admin-stripprefix"]
+
+    [http.middlewares]
+      [http.middlewares.admin-stripprefix.stripPrefix]
+      prefixes = ["/admin"]
+    
+    # ...
+    ```
+
+    ```yaml tab="File (YAML)"
+    ## Dynamic Configuration
+    # dynamic-conf.yml
+
+    # As YAML Configuration File
+    http:
+      routers:
+        admin:
+          service: admin-svc
+          middlewares:
+            - "admin-stripprefix"
+          rule: "Host(`company.org`) && PathPrefix(`/admin`)"
+
+      middlewares:
+        admin-stripprefix:
+          addPrefix:
+            prefix: "/admin"
+
+    # ...
+    ```
+
+??? question "What About Other Path Transformations?"
+
+    Instead of removing the path prefix with the [`stripprefix` middleware](../../middlewares/stripprefix/), you can also:
+
+    * Add a path prefix with the [`addprefix` middleware](../../middlewares/addprefix/)
+    * Replace the complete path of the request with the [`replacepath` middleware](../../middlewares/replacepath/)
+    * ReplaceRewrite path using Regexp with the [`replacepathregex` middleware](../../middlewares/replacepathregex/)
+    * And a lot more on the [`middlewares` page](../../middlewares/overview/)
+
 ## ACME (LetsEncrypt)
 
 [ACME](../https/acme.md) is now a certificate resolver (under a certificatesResolvers section) but remains in the static configuration.
@@ -749,7 +880,7 @@ For a basic configuration, the [metrics configuration](../observability/metrics/
     --metrics.prometheus.entrypoint="metrics"
     ```
 
-## No more root level key/values
+## No More Root Level Key/Values
 
 To avoid any source of confusion, there are no more configuration at the root level.
 Each root item has been moved to a related section or removed.
@@ -976,7 +1107,7 @@ Supported [providers](../providers/overview.md), for now:
 * [x] Rest
 * [ ] Zookeeper
 
-## Some Tips You Should Known
+## Some Tips You Should Know
 
 * Different sources of static configuration (file, CLI flags, ...) cannot be [mixed](../getting-started/configuration-overview.md#the-static-configuration).
 * Now, configuration elements can be referenced between different providers by using the provider namespace notation: `@<provider>`.