docs: add the reverse proxy cache guide

[NicolasBarbey]

Adds a page to the getting started section, "Reverse proxy cache", for the shared cache of thelia/thelia#4148 (Thelia 3.3). It is only in the next version of the docs, since the feature is not released.

The page covers the two settings, what is shared and what is not, the keys a page carries, what purges what, the environment variables of the purge, a Varnish configuration for the enabled mode and another for announce, how to check that it works, what is not purged and the limits. A warning explains why a proxy must drop the forwarded headers a visitor sent, and another why announce is only safe with a proxy that removes the session cookie.

The Varnish configurations were run against Varnish 7.6 with a stub backend: hit and miss, requests with a cookie, a PURGE by key, the refusal of a purge without the secret, and the removal of X-Forwarded-Prefix and Forwarded. Souin was tried as the standalone image: it stores and serves the pages with their keys, but its purge was not demonstrated, and the page says so. The generic surrogate key purger was not tried against a proxy. The page also names two things that keep a page personal: a module that reads a resource outside the list (HeaderHighlights on the demo) and the filter form of a category, whose Flexy fix is in thelia-templates/flexy#288.

The "Application cache" page now links to the new one in place of saying that the HTTP cache is configured only on the proxy. npm run build passes; it reports three broken anchors in pages of version 2.x that this PR does not touch.