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.rst

@@ -0,0 +1,38 @@
+themis package
+==============
+
+.. automodule:: themis
+   :members:
+   :show-inheritance:
+   :undoc-members:
+
+Subpackages
+-----------
+
+.. toctree::
+   :maxdepth: 4
+
+   themis.api
+   themis.templatetags
+
+Submodules
+----------
+
+.. toctree::
+   :maxdepth: 4
+
+   themis.adapters
+   themis.admin
+   themis.apps
+   themis.context_processors
+   themis.encryption
+   themis.forms
+   themis.middleware
+   themis.models
+   themis.notifications
+   themis.signals
+   themis.ssl_patch
+   themis.urls
+   themis.utils
+   themis.views
+   themis.widgets