Fix dead relative link to versioning docs in bump-version skill

[latent-9]

The bump-version skill links to the versioning policy with a relative path that does not exist:

[versioning documentation](../../../../docs/versioning.html)

From .github/skills/bump-version/references/, that resolves to docs/versioning.html at the repo root, and there is no such file. docs/ holds the DocFX sources (CONTRIBUTING.md: "This project uses DocFX to generate its conceptual and reference documentation"), so .html is build output that never gets committed. The only file there is docs/versioning.md, so the link 404s on GitHub.

The .html extension looks like it was copied from line 7 of the same file, which correctly uses the absolute site URL https://csharp.sdk.modelcontextprotocol.io/versioning.html. That line is left untouched.

Two other skill files already refer to the policy by its repo path, both writing it as docs/versioning.md:

• .github/skills/breaking-changes/references/classification.md:68
• .github/skills/prepare-release/SKILL.md:183

Only the relative link is changed.