📚 Docs: Link to GitHub repo from header

The MkDocs site for the template repo had no link back to the source on GitHub,
so readers landing on the docs had to manually navigate to the repo to file an
issue or browse the code.

Wire up Material's built-in repo integration in `mkdocs.yml`: set `repo_url` and
`repo_name`, and pick the GitHub icon for the header link.

Author: mbercx

mkdocs.yml

@@ -1,6 +1,10 @@
 site_name: Python copier
+repo_url: https://github.com/mbercx/python-copier
+repo_name: mbercx/python-copier
 theme:
   name: material
+  icon:
+    repo: fontawesome/brands/github
 markdown_extensions:
   - admonition
   - pymdownx.details