Add cross-file anchor link check

[lmolkova]

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See https://github.com/open-telemetry/semantic-conventions/issues/1008#issuecomment-2097036448

With docfx v3 disappearing into thin air, we have no tooling that validates that relative links to files in the repo with anchors exist (e.g. /docs/http/http-spans.md#setting-serveraddress-and-serverport-attributes)

• markdownlint validates anchors within the same file (#setting-serveraddress-and-serverport-attributes). Same as docfx v2
• markdown-link-check validates presence of file, but not the anchor there

https://github.com/theoludwig/markdownlint-rule-relative-links seems to be able to validate cross-file anchors, but only with relative path (./docs/http/http-spans.md#setting-serveraddress-and-serverport-attributes) which we can't do because of md generation.