docs: migrating between Renovate users/apps

[jamietanna]

Describe the proposed change(s).

I'd like for us to have something "official" documented (other than on my blog 😉) for what folks need to take into account when migrating between users/apps/etc (either Self-Hosted -> Cloud, vice versa, or Self-Hosted -> Self-Hosted)

For teaching the new about the old:

• gitIgnoredAuthors (docs)
• ignorePrAuthor (docs) - this is the one that has impacts to GitHub API rate limits as it removes server-side filtering
• (more reading)

How long shall we recommend to keep "in migrating mode"?

• Personally, 2-4 weeks
• Then remove config + close out old PRs/Issues

When closing:

• (via script) you'll need to make sure you add a suffix to the PR title i.e. - autoclosed so Renovate knows that it can re-raise them in the future
• you'll also want to make sure branches are deleted, otherwise Renovate won't run-