Agent Skills: Document Agent Skills Feature and Create Tutorials

[NTaylorMullen]

Provide comprehensive documentation and tutorials to guide users and skill authors in creating and utilizing Agent Skills effectively within Gemini CLI.

Requirements:

• Feature Overview: Create docs/cli/skills.md to explain the concept, benefits, and progressive disclosure model.
• Authoring Guide: Provide technical guidance on creating a skill directory, including SKILL.md frontmatter requirements and directory structure conventions (scripts/, references/).
• Getting Started Tutorial: Implement a step-by-step tutorial (e.g., docs/cli/tutorials/skills-getting-started.md) that walks a user through creating their first functional skill.
• Index Integration: Update the CLI, Tools, Extensions, and Examples documentation indices to reflect the new feature and point to relevant guides.

Part of #15327

[NTaylorMullen]

We should add this to the docs:

• /skills
  • Description: List, enable, or disable agent skills. Agent skills are
    discovered expertise (via SKILL.md files) that provide specific
    capabilities to the Gemini model.
  • Sub-commands:
    • list
      • Description: Lists all discovered skills and their current status
        (Available vs. Disabled).
      • Usage: /skills list [nodesc]
    • disable
      • Description: Disables a skill, preventing it from being used.
      • Usage: /skills disable <skill-name>
    • enable
      • Description: Re-enables a previously disabled skill.
      • Usage: /skills enable <skill-name>
  • Note: Disabling or enabling a skill requires a session restart to take
    effect.

Feedback: #15699 (comment)