Highlight link to Actuator API docs in the reference docs

[cdalexndr]

Users that fast search this doc may miss that there is another document explaining endpoints api.
The api link is not accessible from main doc reference (https://docs.spring.io/spring-boot/docs/2.4.3/reference/html/), and it is scattered through this doc in 3 places.

This change helps highlight the API link into a header so that it is more visible.

[wilkinsona]

Thanks for the pull request. I agree that we should make the link to the Actuator API documentation more prominent, but I don't think this is how we should do it. I'd rather make the following paragraph more prominent:

To learn more about the Actuator's endpoints and their request and response formats, please refer to the separate API documentation ({spring-boot-actuator-restapi}/html/[HTML] or {spring-boot-actuator-restapi}/pdf/spring-boot-actuator-web-api.pdf[PDF]).

Perhaps it could be made a TIP and/or moved to earlier in the section so it doesn't get lost beneath the table. Would you like to update your PR to that effect?

[cdalexndr]

@wilkinsona what about the other 2 links pointing to the same api doc?

Moving it below header and highlighting it seems a good compromise.

[wilkinsona]

I think the other two links are fine as they are.

[cdalexndr]

@wilkinsona done

[wilkinsona]

Thanks very much, @cdalexndr. I moved the tip down slightly (still before the tables) so that the reader has a bit more context before coming to it.