Add link check to ensure reference exclude anchors to page title/level-1 headings

[chalin]

There is a recurrent problem. GH-repo hosted pages can link to any heading. In such pages, the first heading (which is at level 1) of a page is interpreted as the page title. When the spec gets publish to the OTel website, the level-1 headings are stripped, and instead the title is displayed using specially formatted HTML. The net effect is that there is no anchor in the page for the level-1 heading, because the level 1 heading isn't there anymore.

As an example:

• This illustrates a link to a level-1 heading: K8s naming convention rational and additional documentation #2252 (comment)
• This is the build failure that results: https://github.com/open-telemetry/opentelemetry.io/actions/runs/15373995491/job/43256707459?pr=6948

We should find a way to test this. Maybe a custom markdown link check, or even markdown lint check?