[8.16] Fix API docs link (backport #1415)

[mergify]

This updates the first paragraph of the Kibana Fleet APIs page to fix the broken link and point to the new generated API docs here.

Closes: #1379

Note: I've left the examples on the page in place, but if these should be revised or moved, I'm happy to do that (separately from this PR).

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This is an automatic backport of pull request #1415 done by Mergify.

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