docs: Add link to contributing to docs

On the page for contributing to the docs, there is no direct link
to the docs source code.  This change adds a link to the top of
that page.

doc/rtd/development/contribute_docs.rst

@@ -10,7 +10,7 @@ Contribute to our docs
     Style guide <style_docs.rst>
     Directory layout <docs_layout.rst>
 
-The documentation for cloud-init is hosted in GitHub and rendered on
+The documentation for cloud-init is hosted in `GitHub`_ and rendered on
 `Read the Docs`_. It is mostly written in reStructuredText.
 
 The process for contributing to the docs is largely the same as for code,
@@ -65,6 +65,7 @@ reviewer.
 
 .. LINKS
 .. include:: ../links.txt
+.. _GitHub: https://github.com/canonical/cloud-init/tree/main/doc/rtd
 .. _Read the Docs: https://readthedocs.com/
 .. _tools/.github-cla-signers: https://github.com/canonical/cloud-init/blob/main/tools/.github-cla-signers
 .. _tagging s-makin: https://github.com/s-makin