Document Component Template support

[urso]

PR #21212 did introduce support for component templates. Although the setting is documented in the reference configurations, it is missing from the Documentation.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[an-tex]

Does not apply any more 405c342

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!