Add security warning to Developer docs with dummy links

This is a sub-issue of https://github.com/elastic/docs-content/issues/1734, and the aim is to address the possibility of malicious dummy links in docs by reviewing the related Developer docs, and adding a warning where necessary.

### Parent issue description

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: /
```