Add install directory to docfx.json file references

[flipthedog]

Description

Fixed broken link for https://github.github.com/spec-kit/install/uv.md

Testing

Documentation bug fix

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Copilot suggested *.md

[Copilot]

Pull request overview

Updates the DocFX build configuration so markdown files under docs/install/ are included in the published documentation, addressing a broken /install/uv.md link in the generated site.

Changes:

• Added install/*.md to the DocFX build.content.files include list.
• Adjusted the reference/*.md entry to keep the JSON list syntactically correct after adding the new glob.

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