Add Security Warning to links in Docs

Related to: https://github.com/elastic/apm-agent-rum-js/issues/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". Please let me know if this is the correct repo.