Update Relevant Documentation for incoming Puma switchover

With Puma performance now validated to be within expectations (#64705 (closed)) the next steps are to define what the specific recommendations will be got Puma going forward (omnibus-gitlab#3896 (closed)) and update all relevant documentation to reflect this as the general switch from Unicorn to Puma.

This issue is for updating the docs. E.g. Switching any references to Unicorn to Puma.