docs: add Sphinx documentation build and deploy workflow

- Add Gitea Actions workflow to build and deploy docs on push to main
- Generate Sphinx reference documentation for all apps and modules
- Deploy versioned and latest docs via rsync over SSH

docs/source/index.rst

@@ -0,0 +1,17 @@
+Mnemosyne Documentation
+=======================
+
+Content-type-aware, multimodal personal knowledge management system.
+
+.. toctree::
+   :maxdepth: 2
+   :caption: API Reference
+
+   reference/apps/index
+
+Indices
+-------
+
+* :ref:`genindex`
+* :ref:`modindex`
+* :ref:`search`