[docs] SQL Server HA failover

[sjwiesman]

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[martykulma]

I vote we add a disclaimer about async vs. sync availability mode to let users know about the potential for inconsistency with async.

[kay-kim]

Could we link to the HA section from the note in the ingest data section (that way we let people know that it exists and that the
https://github.com/MaterializeInc/materialize/pull/33747/files#diff-e5e71ad1b8a944d3f24689fefa0d148dd43bc4842ebbf56d59e958281d1061baR241

[sjwiesman]

Thank you both. Addressed.

[tylerhartwig]

If I'm reading this section correctly, manual intervention may be needed in the case that there are only 2 replicas in an AG. (or if all but one database fails). This may be worth calling out as well.

[sjwiesman]

@tylerhartwig that was part of a section as containing best practices. I want to avoid copying to much of microsofts docs over.