Better location for docs for updating pnpm dependencies

[kris-brown]

There is a tricky error one gets after updating one's pnpm dependencies which is addressed by info in the automerge-doc-server README (namely, temporarily replacing the hash with pkgs.lib.fakeHash then getting the correct hash from the ensuing error). However, this issue is relevant at a broader level, so this documentation should live somewhere more easily found.

[epatters]

Additional context: I went looking for this information in the frontend and infrastructure READMEs, then gave up when I couldn't find it in either of those places.