CAPI+CAPA provider upgrade docs

[rudimk]

What type of PR is this?

/kind documentation What this PR does / why we need it:

At the moment, we don't have a doc in the CAPA book that provides an overview of prerequisites needed for upgrading CAPA versions, as well as the actual upgrade process itself. Note - this PR supersedes #4269 - that PR's branch on my end got fouled up. cc @Ankitasw @Skarlso

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #4268

Special notes for your reviewer:

Checklist:

• [X] squashed commits
• [X] includes documentation
• [ ] adds unit tests
• [ ] adds or updates e2e tests

Release note:

Add documentation to the Cluster API AWS book on running provider version upgrades.