refactor: stand up vantage-docs as its own repository

docsite/ becomes the repository root; the whole repository is the site.
The image is vantage/vantage-docs:latest, was mrhid6/vantage/docsite.

The workflow passes no build args, as the monorepo's did not: DOCS_URL,
DOCS_BASE_URL, APP_URL and HQ_URL have correct defaults in the
Dockerfile, and passing an unset repo variable would blank one rather
than leave it alone.

.gitignore

@@ -1,4 +1,4 @@
 node_modules
 build
 .docusaurus
-.cache-loader
+.env