Add the Guides section for the released guide set

[integernick]

The firmware repository gains a released guide set (docs/guides/, mirrored here by scripts/mirror-specs.sh --set guides into src/content/docs/guides/ at every release). This PR prepares the site for it.

• astro.config.mjs: a Guides sidebar section between Getting Started and NXS; redirects from /reference/nxs-driver-development/ and /reference/nxs-cam-descriptors/ to the renamed pages (nxs-patch-authoring, nxs-camera-patches) that arrive with the next release.
• src/content/docs/guides/index.md: the section's hand-authored landing page, which the mirror preserves.
• src/content/docs/reference/index.md: the document list names the seven released documents.
• src/content/docs/index.mdx: a Guides card and a pointer sentence; "driver" reads "patch" where it names the sensor add-on.
• README.md: the guides directory is machine-managed like the reference.
  Not in this PR, because the token that opened it has no workflow scope: the same pre-release gate for the guides in .github/workflows/deploy.yml, in both the main and the staging steps, right after the reference gate's fi:

          if grep -lq "at pre-release" src/content/docs/guides/*.md 2>/dev/null; then
            find src/content/docs/guides -name '*.md' ! -name 'index.md' -delete
            echo "guides seed is pre-release - hidden from the site root"
          fi

Merge before the next rc export: the export's guides mirror step writes into src/content/docs/guides/ and expects this landing page beside it.

Companion: aliensense/redbrain#197.

[github-actions]

✅ Docs preview build success.

Workflow run: https://github.com/aliensense/nxs-docs/actions/runs/34193849042