Adds a breadcrumb template to manage hiding 'Edit on Github' link.

To work, requires adding a meta the very beginning of files that have to hide this link:
:github_url: hide

_templates/breadcrumbs.html

@@ -0,0 +1,7 @@
+{%- extends "sphinx_rtd_theme/breadcrumbs.html" %}
+
+{% block breadcrumbs_aside %}
+{% if not meta or meta.get('github_url') != 'hide' %}
+{{ super() }}
+{% endif %}
+{% endblock %}