docs: add a maintenance-risk (DM001) documentation section

[sonukapoor]

Note: this is an in-house item already being handled by the maintainer - not open for contribution. Filed for tracking only.

--check-maintenance / DM001 shipped in #854 but has no documentation on the website - it isn't even in cli-reference.md. Meanwhile the sibling --check-overrides has a full override-hygiene/ section (an index page plus one page per rule: OA001-OA009, PD001-PD002), wired into the sidebar. The maintenance-risk family should get the same treatment, especially since it's set to grow (DM002 staleness #835, DM003 GitHub archive #836, and a planned cooldown-aware rule).

What to do

• Create a maintenance-risk/ docs section mirroring override-hygiene/:
  • maintenance-risk/index - the concept: what maintenance risk / constraint drag / abandonware is, why CVE scanners miss it, how --check-maintenance works, and where it fits alongside CVE and override findings.
  • maintenance-risk/dm001 - the DM001 rule page (constraint drag + npm-deprecated detection), same shape as the oa00N rule pages.
• Add the missing --check-maintenance entry to website/docs/cli-reference.md (usage block + options), mirroring how --check-overrides appears.
• Wire both new pages into website/sidebars.ts, right after the override-hygiene section.
• Add a one-line --check-maintenance mention to README.md alongside --check-overrides.
• Cross-link the two sections for symmetry.

Notes

• Run npm run build in website/ to confirm the Docusaurus site builds and there are no broken links.
• Keep DM001 semantics accurate: it flags a direct dep that pins a transitive dep below its CVE fix (constraint drag, needing a major-version parent upgrade), or that is deprecated on npm. The rendered detail distinguishes the constrained package's fix version from the parent's upgrade target.