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 🤔

[wWzZb]

Agreed — absolute published URLs are not the right fix because they degrade the GitHub browsing experience.

I reproduced the underlying page.md#fragment failure on the latest phpDocumentor/guides main using this SDKs actual mcp-elements.md → server-builder.md#... links. The upstream fix is phpDocumentor/guides#1300. I refreshed it onto current main, confirmed PHP 8.5 lowest dependencies and the full locked suite pass, and added regression coverage for the PHP SDK link shape. Since I cannot update the contributor branch directly, the clean mergeable refresh is at jonathanhefner/phpDocumentor-guides#2.

I am closing this workaround in favor of the upstream fix. Issue #232 should remain open until that fix is merged and available to the docs build.