[Docs] Add details to SO APIs landing page

[TinaHeiligers]

Fix #191534
The deprecation warning in the Kibana Saved Objects API main page was removed in #190654.

This PR adds a section about when the APIs will be removed and what to use instead.

The PR also includes updates to other API main pages that mention the SO APIs.

• Documentation was added for features that require explanation or tutorials

VIew the html diff for these changes:

The updated OpenAPI document looks like this in a Bump.sh preview:

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[TinaHeiligers]

FYI @elastic/kibana-core

[mattkime]

changes lgtm

[lcawl]

LGTM, thanks!

[TinaHeiligers]

@elasticmachine merge upstream

[kibana-ci]

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