docs: fix guide fragment links

[wWzZb]

What

• Replace three broken cross-page fragment links in docs/mcp-elements.md with canonical published guide URLs.
• Keep the same target sections while making the links work in both GitHub Markdown and generated documentation.

Why

phpDocumentor does not resolve Markdown paths that include fragments, so the generated links disappear. Canonical absolute URLs preserve the fragments without changing the documentation layout.

Impact

Documentation-only change; no runtime behavior is affected.

Checks

• make docs

Closes #232

[chr-hertel]

Unfortunately absolute links break the experience in GitHub - I think unless the upstream issue is not solved and we don't switch to RST, we'd need to switch the tooling 🤔