WIP: update multi-tenacy docs

[davidblum]

This PR updates the multi-tenancy section to provide:

• syntax fixes
• more explanation around components
• an applyable example

/kind documentation

What this PR does / why we need it:

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

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Release note:

[LucaLanziani]

I believe this might be redundant, the identityRef has nothing to do with the account per se, and the paragraph at line 24 clarify what is this about.

[davidblum]

I understand the point you're making. I think while it may be redundant, it states an important part of the multi-tenancy configuration. Which is how an operator dictates which cluster they can deploy into.

I'd like to leave something high level explaining this at the top of the doc, do you have any suggestions for wording?

[LucaLanziani]

You could say something like:

identityRef reference the credentials or the role that the capa-controller will use to manage the relative cluster

But then we are, sort of, repeating what's in the previous paragraph

[LucaLanziani]

Aside from the comment above, LGTM