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/configuring_tisbackup.rst

@@ -82,7 +82,7 @@ Backing up a MySQL database
   [srvintranet_mysql_mediawiki]
   type=mysql+ssh
   server_name=srvintranet
-  private_key=/root/.ssh/id_dsa
+  private_key=/root/.ssh/id_ed25519
   db_name=mediawiki
   db_user=user
   db_passwd=password
@@ -141,7 +141,7 @@ Backing up a file server
   type=rsync+ssh
   server_name=srvfiles
   remote_dir=/home
-  private_key=/root/.ssh/id_dsa
+  private_key=/root/.ssh/id_ed25519
   exclude_list=".mozilla",".thunderbird",".x2go","*.avi"
   bwlimit = 100