Add Prettier auto-formatting for docs content, and remove docs-lint-markdown

[alexandratran]

Context

Prettier is already configured in this repo (.prettierrc, format script, lint-staged block) but never enforced — there's no .husky/prepare script wiring it to git hooks, and CI's docs-lint-all step only lints .js/.ts/.jsx/.tsx (ESLint) and .css (Stylelint); eslint.config.mjs explicitly ignores .md/.mdx/.json. So doc content has zero formatting enforcement today.

Separately, lint.yml's docs-lint-markdown step is currently a no-op: it defaults to checking docs/**/*.md, but docs/ has 294 .mdx files and zero .md files, so it never matches anything and silently passes without checking content.

Proposal

1. Add a Prettier CI check scoped to changed files only (see metamask-docs's ci.yml prettier job for the pattern).
2. Wire up .husky + prepare so lint-staged actually runs pre-commit locally.
3. Remove the docs-lint-markdown step from lint.yml.
4. Review and update .prettierrc for docs before turning any of this on — proseWrap and printWidth in particular will almost certainly need to change from their current defaults (always / 80) to match how Linea docs are actually written, or the first run will force a massive rewrap diff across nearly every page.

Notes

• Needs its own investigation into the right Prettier config for this content (proseWrap, printWidth, frontmatter quote style, MDX-specific quirks) — don't assume current .prettierrc values are right for docs.
• Decide file-type scope for the new CI check (at minimum .md/.mdx; avoid re-checking .js/.ts/.jsx/.tsx/.css, already covered by existing ESLint/Stylelint).