Update documentation on supported deployment methods

[eeaton]

TL;DR

Before we can do the larger piece of work in #1249, we need a quick documentation fix to signpost which deployment methods are supported for which scenarios.

Things to

Terraform Resources

No response

Detailed design

Things to fix:
 - Current default only works if your org has previously used CSR. https://cloud.google.com/source-repositories/docs. If not, you must use one of the other options (local, GitHub, Gitlab)
 - We no longer maintain the jenkins directions, we've left the legacy instructions available but make no guarantees.

Additional information

No response

[eeaton]

Address by PR #1276.