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/reference/apps/themis/themis.api.rst

@@ -0,0 +1,17 @@
+themis.api package
+==================
+
+.. automodule:: themis.api
+   :members:
+   :show-inheritance:
+   :undoc-members:
+
+Submodules
+----------
+
+.. toctree::
+   :maxdepth: 4
+
+   themis.api.serializers
+   themis.api.urls
+   themis.api.views