docs: add skill for updating Next.js documentation (#88656)
### What?
Adds a new Claude Code skill `/update-docs` that guides maintainers
through updating Next.js documentation based on code changes.
### Why?
To help ensure PRs include complete and accurate documentation updates
by providing a structured workflow, code-to-docs mappings, and quick
reference materials for Next.js doc conventions.
### How?
The skill includes:
- Main workflow in `SKILL.md` with step-by-step guidance for analyzing
changes, updating docs, and scaffolding new feature documentation
- `references/DOC-CONVENTIONS.md` - Complete frontmatter schema, code
block formatting, and MDX component usage
- `references/CODE-TO-DOCS-MAPPING.md` - Maps source code directories to
their corresponding documentation paths
### Usage
**Manual invocation:**
```
/update-docs
```
**Automatic triggers** - the skill activates when you mention:
- "update documentation for my changes"
- "check docs for this PR"
- "what docs need updating"
- "sync docs with code"
- "scaffold docs for this feature"
- "document this feature"
- "review docs completeness"
- "add docs for this change"
- "what documentation is affected"
- "docs impact"
- References to `docs/`, `docs/01-app`, `docs/02-pages`, or `.mdx files`
