doc: add pilot and plugins documentation.

docs/content/plugins/using-plugins.md

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+# Using Plugins
+
+Since v2.3, plugins are available to any Traefik instance that is [registered](overview.md#connecting-to-traefik-pilot) with Traefik Pilot.
+Plugins are hosted on GitHub, but you can browse plugins to add to your registered Traefik instances from the Traefik Pilot UI.
+
+!!! danger "Experimental Features"
+    Plugins can potentially modify the behavior of Traefik in unforeseen ways.
+    Exercise caution when adding new plugins to production Traefik instances.
+
+## Add a Plugin
+
+To add a new plugin to a Traefik instance, you must modify that instance's static configuration.
+The code to be added is provided by the Traefik Pilot UI when you choose **Install the Plugin**.
+
+In the example below, we add the [`blockpath`](http://github.com/containous/plugin-blockpath) and [`rewritebody`](https://github.com/containous/plugin-rewritebody) plugins:
+
+```toml tab="File (TOML)"
+[entryPoints]
+  [entryPoints.web]
+    address = ":80"
+
+[experimental]
+  [experimental.pilot]
+    token = "xxxxxxxxx"
+
+  [experimental.plugins]
+    [experimental.plugins.block]
+      modulename = "github.com/containous/plugin-blockpath"
+      version = "v0.1.2"
+    [experimental.plugins.rewrite]
+      modulename = "github.com/containous/plugin-rewritebody"
+      version = "v0.2.0"
+```
+
+```yaml tab="File (YAML)"
+entryPoints:
+  web:
+    address: :80
+
+experimental:
+  pilot:
+    token: xxxxxxxxx
+
+  plugins:
+    block:
+      modulename: github.com/containous/plugin-blockpath
+      version: v0.1.2
+    rewrite:
+      modulename: github.com/containous/plugin-rewritebody
+      version: v0.2.0
+```
+
+```bash tab="CLI"
+--entryPoints.web.address=:80
+--experimental.pilot.token=xxxxxxxxx
+--experimental.plugins.block.modulename=github.com/containous/plugin-blockpath
+--experimental.plugins.block.version=v0.1.2
+--experimental.plugins.rewrite.modulename=github.com/containous/plugin-rewritebody
+--experimental.plugins.rewrite.version=v0.2.0
+```
+
+## Configuring Plugins
+
+Some plugins will need to be configured by adding a dynamic configuration.
+For the `bodyrewrite` plugin, for example:
+
+```yaml tab="Docker"
+labels:
+  - "traefik.http.middlewares.my-rewritebody.plugin.rewrite.rewrites[0].regex=example"
+  - "traefik.http.middlewares.my-rewritebody.plugin.rewrite.rewrites[0].replacement=test"
+```
+
+```yaml tab="Kubernetes"
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: my-rewritebody
+spec:
+  plugin:
+    rewrite:
+      rewrites:
+        - regex: example
+          replacement: test
+```
+
+```yaml tab="Consul Catalog"
+- "traefik.http.middlewares.my-rewritebody.plugin.rewrite.rewrites[0].regex=example"
+- "traefik.http.middlewares.my-rewritebody.plugin.rewrite.rewrites[0].replacement=test"
+```
+
+```json tab="Marathon"
+"labels": {
+  "traefik.http.middlewares.my-rewritebody.plugin.rewrite.rewrites[0].regex": "example",
+  "traefik.http.middlewares.my-rewritebody.plugin.rewrite.rewrites[0].replacement": "test"
+}
+```
+
+```yaml tab="Rancher"
+labels:
+  - "traefik.http.middlewares.my-rewritebody.plugin.rewrite.rewrites[0].regex=example"
+  - "traefik.http.middlewares.my-rewritebody.plugin.rewrite.rewrites[0].replacement=test"
+```
+
+```toml tab="File (TOML)"
+[http.middlewares]
+  [http.middlewares.my-rewritebody.plugin.rewrite]
+    lastModified = true
+    [[http.middlewares.my-rewritebody.plugin.rewrite.rewrites]]
+      regex = "example"
+      replacement = "test"
+```
+
+```yaml tab="File (YAML)"
+http:
+  middlewares:
+    my-rewritebody:
+      plugin:
+        rewrite:
+          rewrites:
+            - regex: example
+              replacement: test
+```