Proxito: add top-level docs (#8913)

Co-authored-by: Eric Holscher <[email protected]>

Author: stsewd

readthedocs/proxito/README.rst

@@ -0,0 +1,55 @@
+Proxito
+=======
+
+Module in charge of serving documentation pages.
+
+Read the Docs core team members can view the `Proxito design doc <https://github.com/readthedocs/el-proxito/blob/master/docs/design/architecture.rst>`_
+
+CDN
+---
+
+We use the ``CDN-Cache-Control`` header to control caching at the CDN level,
+this doesn't affect caching at the browser level (``Cache-Control``).
+See https://developers.cloudflare.com/cache/about/cdn-cache-control.
+
+The cache control header is only used when privacy levels
+are enabled (otherwise everything is public by default).
+
+By default, all requests on proxito are marked as private,
+but individual views may mark a request as public.
+This was done since what is considered public varies on each view,
+or the details to know this are only accessible on the view itself
+(like the final project attached to the request).
+
+What can/can't be cached?
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- Footer: should never be cached.
+  We show a different footer depending on the user,
+  even if they are on a public version.
+- ServePageRedirect: can be cached for public versions, or for all versions,
+  as the final URL will check for authz.
+- ServeDocs: can be cached for public versions.
+- ServeError404:
+  This view checks for user permissions, can't be cached.
+
+  We could cache it only:
+  - If the response is a redirect (slash redirect or user redirect) and the version is public.
+  - If current version and the default version are public (when serving a custom 404 page).
+
+- ServeRobotsTXT: can be cached, we don't serve a custom robots.txt
+  to any user if the default version is private.
+  This view is already cached at the application level.
+- ServeSitemapXML: can be cached. It displays only public versions, for everyone.
+  This view is already cached at the application level.
+- Embed API: can be cached for public versions.
+- Search:
+  This view checks for user permissions, can't be cached.
+  Additionally, to the privacy level of the version,
+  we check for authz when including results from subprojects,
+  so search results may be distinct for each user.
+
+  We could cache it only:
+  - If the project doesn't have subprojects.
+  - All subprojects are public.
+- Analytics API: can't be cached, we want to always hit our serves with this one.