Update Docusaurus, lint, site chrome, and docs

[davidnewhall]

Summary

• Pin Docusaurus 3.10.2 with Faster/v4 future flags, React 19.2.8, ESM config, and a refreshed Yarn lockfile (same shape as unpackerr.github.io#47).
• Replace Ruby mdl with markdownlint-cli2, add codespell to the Build workflow, and convert :::tip Title callouts to :::tip[Title] for MDX v2 (#48).
• Fix leftover site chrome and docs facts: navbar GitHub link (was Docusaurus), GHCR image names, Compose v2, Synology UniFi env vars, Helm upgrade target, Homebrew installer, and install-page typos (#49 / #50).
• Replace Dependabot with Renovate: pin Action digests, 7-day minimumReleaseAge, automerge GitHub Actions (including majors) and TypeScript minor/patch after CI passes.
• Reverts Replace Renovate with Dependabot #57 - dependabot could work, but I use renovate on all personal projects because it updates things dependabot doesn't. I don't think this repo has any of those things, but I'm just keeping this consistent with other repos, like the unpackerr one linked above. If you hate it, tell me, and I'll remove it.
• Adjusted the repo rulesets so renovate can auto merge PRs when the build passes.

Test plan

• GitHub Actions Build workflow passes on this PR
• cd documentation && yarn lint && yarn build succeeds locally (already verified)
• Spot-check https://unpoller.com navbar (GitHub, not Docusaurus), Introduction, Docker Compose, Synology env vars, and Kubernetes Helm upgrade
• After merge, confirm the Renovate app is enabled on this repo and opens/automerge PRs
• After merge, deploy workflow publishes to unpoller.com

Made with Cursor

[davidnewhall]

I read through all of this and adjusted a few things along the way. Seems good to me, but I recommend reading through it so we can catch anything dumb that Grok did (already caught a couple).

[davidnewhall]

Published version looks good to me!