fix(docs): fix sponsor link baseURL and add git commit hash to footer

- Use relURL (no leading slash) for the sponsor link so it respects
  the /Bose-SoundTouch/ base path on GitHub Pages; absURL and relURL
  both ignore the base path when the input starts with /
- Inject HUGO_PARAMS_GITHASH (github.sha) via the docs workflow and
  forward it into the Hugo container via docker-compose.docs.yml +
  make dev-docs, so the deployed footer shows a clickable short hash
  linking to the exact commit
- Use site.Params.githash (global) instead of .Site.Params.githash
  because Hextra calls custom/footer.html with a dict context, not a
  page; .Site is nil in that scope
- Use substr not slice to trim the hash to 7 chars

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/docs.yml

@@ -33,6 +33,7 @@ jobs:
         run: hugo --source docs/ --minify --destination ../_site --baseURL "${{ steps.pages.outputs.base_url }}"
         env:
           HUGO_ENVIRONMENT: production
+          HUGO_PARAMS_GITHASH: ${{ github.sha }}
       - name: Upload artifact
         uses: actions/upload-pages-artifact@fc324d3547104276b827a68afc52ff2a11cc49c9 # v5.0.0
         with: