ci(docs): add markdown link check job and fix broken links- Closes #132

[afshan-khan549]

Summary

Adds a docs-link-check CI job (per #132) that runs markdown-link-check on
PRs and weekly against main, and fixes the broken links it found in the
process.

Changes

• .github/workflows/docs-link-check.yml — new workflow running
  gaurav-nelson/github-action-markdown-link-check on PRs targeting main
  and on a weekly schedule
• .github/mlc_config.json — link-checker config: ignores localhost URLs,
  sets a browser-like User-Agent for GitHub URLs, treats 429 as alive
  (rate-limit tolerance), and ignores MkDocs clean-URL-style relative links
  (e.g. getting-started/quickstart/) in docs/index.md, which are valid
  once the site is built via mkdocs build but aren't real raw file paths
• README.md — fixed the "Stars" badge link, which pointed to
  /stargazers, a route that 404s on GitHub for this repo (confirmed also
  404s on torvalds/linux/stargazers, so likely a broader GitHub behavior
  change); now links to the repo root instead
• SUPPORT.md — removed the "Community" section linking to GitHub
  Discussions, which is disabled for this repo (has_discussions: false
  via the GitHub API)

Testing

Ran markdown-link-check locally against every .md file in the repo
(via npx markdown-link-check --config .github/mlc_config.json) before
and after the fixes:

• Before: 2 genuinely broken links (README.md stargazers badge,
  SUPPORT.md discussions link) + 13 false-positive failures on MkDocs
  clean URLs in docs/index.md
• After: 0 dead links across all ~50 markdown files in the repo

No Python code was changed, so ruff/mypy/pytest/bandit are not
applicable to this PR.

Checklist

• Branch named per Git Workflow (ci/docs-link-check-132 — following
  the ci commit-type convention, since a dedicated ci/ branch prefix
  isn't listed in the branch-naming table)
• Not developed on main
• Code follows AGENT.md coding standards — N/A, no Python code changed
• domain/ and ports/ stay pure — N/A, no Python code changed
• Quality gates pass locally: ruff, mypy --strict, bandit, pytest — N/A, no Python code changed
• Docs updated if behavior changed — fixed the two broken doc links directly
• pyproject.toml and src/modeldock/__init__.py versions match — N/A, not release-related