[8.x] Add docs for orphaned integrations (backport #1296)

[mergify]

This adds a few sentences to mention orphaned integrations, i.e., those that have been removed from and are no longer associated with any Elastic Agent policies.

@jen-huang Please let me know if I've missed anything.

Closes: #1261
Rel: #1220

This content appears in two spots:

• Add an integration to an Elastic Agent policy (in the "Manage integrations" section of the docs)
• Add an integration to a policy (in the Agent Policies section of the docs)

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This is an automatic backport of pull request #1296 done by [Mergify](https://mergify.com).

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