docs: add comprehensive security and authentication documentation

Add new documentation sections covering security best practices and authentication system architecture. Update Sphinx configuration and dependencies to support documentation improvements.

Changes include:
- New security.rst with SSH key management, network security, secrets management
- New authentication.rst documenting pluggable auth system and provider setup
- Updated Sphinx config to use Alabaster theme and add sphinx-tabs extension
- Added docs extra dependencies in pyproject.toml for documentation builds
- Updated example configs to use Ed25519 instead of deprecated DSA keys
- Added .python-version file for consistent Python version management
- Added CLAUDE.md project instructions for AI-assisted development
- Minor Dockerfile cleanup removing commented pip install line

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs-sphinx-rst/source/index.rst

@@ -92,6 +92,13 @@ would have been difficult to develop as an overlay of the existing one:
   configuring_tisbackup.rst
   using_tisbackup.rst
 
+.. toctree::
+  :maxdepth: 2
+  :caption: Security & Authentication
+
+  security.rst
+  authentication.rst
+
 .. toctree::
   :maxdepth: 1
   :caption: Appendix