Fix documentation 404s and generate static redirects

[floitsch]

Google's 404 report exposed 19 internal links pointing to 17 missing documentation destinations. Correct those links across 13 pages using root-relative paths, including the MQTT setup links, cellular container links, PWM LED paths, and malformed tasks link.

Make static/redirects.yaml effective on the current GitHub Pages deployment by generating 28 standalone HTML redirect pages and registering both trailing-slash variants with Gatsby. Direct visits use an immediate meta refresh with a canonical URL; JavaScript preserves query strings and fragments. These are HTML redirects, not HTTP 301 responses. Retired Toit v1 pages without a clear replacement remain 404.

Add yarn check:links to the production build so missing internal destinations fail the build. The check uses Python 3 and inspects generated HTML without network access; external links and fragment identifiers are excluded. Redirect validation rejects existing-page collisions, missing targets, and redirect chains.

Validation:

• Production HTML generation completed; yarn check:links passes for all 139 HTML files.
• yarn lint passes.
• yarn test --runInBand passes: 3 suites, 10 tests.
• Verified all 28 redirect pages exist, both slash variants are registered, aliases are absent from the sitemap, and current content links directly to the replacement pages.
• Link-checker fixtures cover valid pages/assets, external links, fragments, missing destinations, and a missing build.