Add Security Warning to links in Docs

@shainaraskas jun 30: for security reasons, we plan to replace any placeholder/fake URLs with non-hostable placeholder segments.

Remove segments that make a valid URL, including prefixes (`http://`) and TLDs (`.com`).

before:
```

```
after:
```

```

The placeholder `<>` can be adapted to use descriptive text for its purpose. For example, a placeholder for a splunk server might look like this:

```
server: /
```

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original content:

> 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.