docs: Update README for the repository itself

Adds a lot more information about what is actually going on here, as
well as links to relevant things such as our Monorepo document.

I'm aware that I didn't make all `//...` links clickable, but I
realised at some point that it might be easier to just update cheddar
to support highlighting those natively :-)

Change-Id: Icbb212a6c07a5cf38ab8e65d83a64bec783eb8d0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1768
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: kanepyork <rikingcoding@gmail.com>

ops/besadii/default.nix

@@ -1,5 +1,5 @@
-# This program is used as a git post-update hook to trigger builds on
-# sourcehut.
+# This program is used as a Gerrit hook to trigger builds on
+# Buildkite, Sourcegraph reindexing and other maintenance tasks.
 { ciBuilds, depot, ... }:
 
 let