MNT: restore ability to put the sha of the docs in the output

The commit the docs are built from may be different from the commit of the
installed library.  This is always the case for our published documentation.

Author: tacaswell

mpl_sphinx_theme/__init__.py

@@ -31,7 +31,10 @@ def mpl_path(path):
 # For more details, see:
 # https://www.sphinx-doc.org/en/master/development/theming.html#distribute-your-theme-as-a-python-package
 def setup(app):
-    app.add_html_theme("mpl_sphinx_theme",
-                       str(Path(__file__).parent.resolve()))
+    here = Path(__file__).parent.resolve()
+    # Include component templates
+    app.config.templates_path.append(str(here / "components"))
+
+    app.add_html_theme("mpl_sphinx_theme", str(here))
     app.connect("html-page-context", setup_html_page_context)
     return {'version': __version__, 'parallel_read_safe': True}

mpl_sphinx_theme/components/lib_version.html

@@ -0,0 +1,6 @@
+{% if lib_version %}
+  <p class="sphinx-version">
+    Built from {{ lib_version }}.
+    <br/>
+  </p>
+{% endif %}