Adds documentation for the new Skills feature, which replaces Rules
starting in Zed v1.4.0.

## Changes

- **New page**: `docs/src/ai/skills.md` — covers what skills are,
finding and installing skills from skills.sh, using skills via slash
commands and model invocation, creating a skill (folder structure,
SKILL.md format, frontmatter fields, bundled resources), where skills
live (global vs project-local), security model, and limitations
- **Updated**: `docs/src/ai/rules.md` — added deprecation notice
pointing to Skills
- **Updated**: `docs/src/SUMMARY.md` — added Skills entry above Rules in
the AI section
- **Updated**: `docs/src/ai/agent-panel.md` — replaced reference to
"rules files" with "skills" in the Adding Context section

Release Notes:

- Added documentation for the Skills feature

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Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>