Update the extension documentation so it's clear that extensions are deprecated
Created by: philipp-spiess
We have a couple of options:
- Keep the extensions as-is with the deprecation notices
- Delete the documentation. However, when we do that, users that are on pre-4.0 instances are also no longer able to easily see the documentation (as public links on sourcegraph.com won't work anymore).
- Change the documentation to make it clear that this is only enabled with
enableLegacyExtensions - Delete the documentation but have a new doc page added about
enableLegacyExtensionswith the most useful information from the previous docs
cc @sourcegraph/integrations
/cc @muratsu @jjinnii @ryankscott