Bug 1654339 - Add Glean metrics autodocs to moztreedocs r=janerik,ahal

Also define a scheme for storing the index of Glean definitions files in a file
separate from the build system for consumption by
* mach build
* mach doc
* (future) mozilla/probe-scraper

Differential Revision: https://phabricator.services.mozilla.com/D87600

docs/conf.py

@@ -27,6 +27,7 @@ EXTRA_PATHS = (
     "third_party/python/futures",
     "third_party/python/jsmin",
     "third_party/python/which",
+    "toolkit/components/glean/sphinx",
 )
 
 sys.path[:0] = [os.path.join(topsrcdir, p) for p in EXTRA_PATHS]
@@ -46,6 +47,7 @@ extensions = [
     "recommonmark",
     "sphinx_copybutton",
     "sphinx_markdown_tables",
+    "glean",
 ]
 
 # JSDoc must run successfully for dirs specified, so running