[DPE-7242] Add multi-cluster refresh docs

[a-velasco]

Issue

Multi-cluster deployment upgrades require some special considerations that are not documented.

Solution

Updated the "Upgrades" section of the documentation to include information about multi-cluster upgrades:

• Added new multi-cluster page
• Updated the taxonomy: page titles clarify if they refer to single vs. multi-cluster, and removed confusing "major/minor" version terminology
• Updated Upgrades landing page accordingly

Additional changes:

• Updated general upgrade terminology to "refresh" for consistency with Juju and other Data charms
• Did some minor polishing of the content in existing upgrade/refresh guides
• Updated cross-regional async replication guides to cluster-cluster terminology
  • A more in-depth refactor of the content is planned for a separate task

TODO:

• Resolve "TODOs" in upgrade guides

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[paulomach]

quick read, found some points

[sinclert-canonical]

Just chatted with Alex about the single/multi-cluster structure of docs. Apologies for the back-and-forth @a-velasco

He seems happy about dropping the minor upgrades terminology, as that is not something applicable to MySQL, however, he will rather have the upgrade and rollback pages within their respective collapsible sections. Reason being: whenever users face a rollback situation, they may be anxious about the state of their cluster, and follow the instructions from the wrong page. Indenting the pages within their respective folders is more error-proof.

• Single-cluster:
  • Upgrade
  • Rollback
• Multi-cluster:
  • Upgrade
  • Rollback

We can chat about it on Tuesday meeting.

[paulomach]

Multi cluster setup is good, thanks.