Documentation Revamp

Co-authored-by: jbdoumenjou <jb.doumenjou@gmail.com>

docs/content/middlewares/stripprefixregex.md

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+# TODO -- StripPrefix
+
+Removing Prefixes From the Path Before Forwarding the Request (Using a Regex)
+{: .subtitle }
+
+## OldContent
+
+Use a `*Strip` matcher if your backend listens on the root path (`/`) but should be routeable on a specific prefix.
+For instance, `PathPrefixStrip: /products` would match `/products` but also `/products/shoes` and `/products/shirts`.  
+Since the path is stripped prior to forwarding, your backend is expected to listen on `/`.  
+If your backend is serving assets (e.g., images or Javascript files), chances are it must return properly constructed relative URLs.  
+Continuing on the example, the backend should return `/products/shoes/image.png` (and not `/images.png` which Traefik would likely not be able to associate with the same backend).  
+The `X-Forwarded-Prefix` header (available since Traefik 1.3) can be queried to build such URLs dynamically.