Add an 'Examples' page to the docs with fully functioning example applications

Nobody has claimed this yet.

There are a few cases of interesting usages that come up in issues, but which don't seem appropriate to put into our main docs (b/c we don't want to bloat our docs). If we had an Examples page in the docs, we could show off all of our cool ideas for how you can solve problems with `webargs`, but without disrupting the flow of the main documentation.

Specific cases which would be good candidates for example apps:
- [Django App with CSRF-Removing Parser](https://github.com/marshmallow-code/webargs/issues/640#issuecomment-930276628)
- [Axios Query-String Parser](https://github.com/marshmallow-code/webargs/issues/517#issuecomment-634061481)
- Setting a parser to use `unknown=RAISE` on query params

I did a quick scan of closed issues until I hit the 5.x/6.0 transition point, then called it a day. Maybe there are other interesting examples in older issues? Or maybe we have things in other parts of the docs like the upgrade guide which would be nice to separate out (or even just duplicate).

Start by reviewing the linked issue comments for the Django CSRF-removing parser and Axios query-string parser, then inspect the existing webargs documentation and upgrade guide for other candidates. Define the Examples page structure and select a focused set of fully functioning applications. Done means the page and runnable examples cover the agreed cases without bloating the main documentation.

Written by the indexing model from the issue text.