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>

docker-compose.docs.yml

@@ -27,6 +27,8 @@ services:
       # downloads Hextra once.
       - hugo-mod-cache:/root/.cache/hugo_cache
     working_dir: /src
+    environment:
+      - HUGO_PARAMS_GITHASH
 
 volumes:
   hugo-mod-cache: