Add guide for changing configuration repository

[carolili]

This PR adds a guide to the documentation for changing the repository containing the configuration that Flux applies.

Contents:

• Setting up a new repository and credentials.
• Defining the new repository setup as Flux resources.
• Pointing the old Kustomization(s) to the new repository.

The examples aim to be generic and contain fake examples of usernames/credentials. Any feedback is welcome.

Please review the documentation especially with regards to the Kustomization and pointing the Kustomization to a new repo to ensure that no important detail is missed.

[dholbach]

Thanks a lot for your PR. I just had a quick pass over it and checked grammar, spelling, etc. I'll need to defer to somebody who knows Flux better for the actual content.

[dholbach]

It looks like @csjiang and @stealthybox are having a discussion about something similar in #187. Stefan said that the two PRs should potentially be merged and maybe not repeat the information from the source-controller docs. Is this something folks would be interested in taking on?

[carolili]

Thank you, I will add the suggested changes so you can proceed. If the docs in #187 are already covering this use case, feel free to mention changing configuration repository as a use case and reject this PR.