docs: added documentation for usage of failure domains

[Ankitasw]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
This PR adds the documentation for usage of failure domains in CAPA for control plane and worker nodes.

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 #2924

Checklist:

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

Release note:

Added documentation for usage of failure domains in control planes and worker nodes

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[sedefsavas]

I think we can combine this with control plane AZ support:
https://cluster-api-aws.sigs.k8s.io/topics/multi-az-control-planes.html
Maybe under a single section Failure Domains, with 2 subsections workers and control-plane. WDYT?

Also, this needs to be added to SUMMARY_PREFIX.md to be added to the book index. You can check locally how changes look by running make serve-book.

[Ankitasw]

I think we can combine this with control plane AZ support:
https://cluster-api-aws.sigs.k8s.io/topics/multi-az-control-planes.html
Maybe under a single section Failure Domains, with 2 subsections workers and control-plane. WDYT?

So do we want to cover single-AZ and multi AZ support in control plane and worker nodes in same doc, i.e, Failure Domains ?

[sedefsavas]

Not the same file but section would be better, otherwise will be too long. My above comment:

Maybe under a single section Failure Domains, with 2 subsections workers and control-plane. WDYT?

[Ankitasw]

Not the same file but section would be better, otherwise will be too long.

@sedefsavas Arranged the sections, please have a look.

[enxebre]

Do we want to elaborate here how .failureDomain and .Subnet relate?
e.g
We have 2 sources for subnets:

1. If subnet.id or subnet.filters are specified, we directly query AWS
2. All other cases use the subnets provided in the cluster network spec without ever calling AWS

Relates to #2864
cc @codablock

[Ankitasw]

It's been present in control-planes. Do we want to add similar explanation for worker nodes as well?

[enxebre]

ah thanks @Ankitasw, I had missed that. My +1 to present it in a way that's clear that apply to both cp and worker nodes as well.

[Ankitasw]

sure @enxebre I will summarize it for both cp and worker nodes.

[Ankitasw]

Added a note here, so that we know it applies for worker nodes as well

[richardcase]

/lgtm

[sedefsavas]

/approve

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