Add a standalone ci workflow so handbook changes get an automated gate.
The job is named literally "ci / ci" to emit the exact status context the
org main-branch-protection ruleset requires.
- markdownlint-cli2 (.markdownlint-cli2.jsonc): MD013/MD060/MD040 relaxed
as noisy docs-style rules; fixed 3 MD032 blank-line-around-list issues.
- lychee link check (lychee.toml): internal + external links, tolerates 429.
