Convert the docs generation custom agent to a skill

[roji]

Easier to use locally and more standard nowadays.

[Copilot]

Pull request overview

This PR updates the metadata and introductory wording for the EF docs-generation agent now represented as a skill, making it easier to invoke consistently in the current .agents/skills/ structure.

Changes:

• Updated the skill front matter (name, description) to match the skill-based packaging.
• Refined the introduction to reference an EF issue and/or PR as the input for generating a docs PR.

Comments suppressed due to low confidence (1)

.agents/skills/generate-docs/SKILL.md:8

• The phrasing “issue and/or PR” (and later “issue/PR”) is ambiguous about what inputs are supported and reads a bit informal. Consider rewording to explicitly say “issue or pull request (PR)” and then consistently use that term throughout the paragraph.