Add security warning to Admin docs with dummy links

[vishaangelova]

This is a sub-issue of #1734, and the aim is to address the possibility of malicious dummy links in docs by adding a warning in the related Admin docs.

Parent issue description

Related to: elastic/apm-agent-rum-js#1611

In recent light of finding malicious actors abusing dummy links in our Docs, we'd like to do the following additional step:

[In the affected piece(s) of documentation, include a conspicuous security warning (e.g. a bright-colored box) advising users to "refrain from following any link to dummy URLs like [include non-clickable image copy of the original dummy URL] because threat actors are known to be misusing such URLs to distribute malware”.

[shainaraskas]

looking into alternatives in the parent issue