Repogroup pages
Created by: attfarhan
Fixes #11555. Fixes https://github.com/sourcegraph/sourcegraph/issues/11537.
This PR implements stage 1 of RFC 159.
This includes an implementation of our new search homepage and the addition of new repository page on Sourcegraph Cloud. Currently, this is in the MVP phase, so all repogroup page content is curated by the Sourcegraph team. Each page has its own file (e.g. web/src/repogroups/Android.tsx), and is configured via a RepogroupMetadata object.