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/installing_tisbackup.rst

@@ -251,14 +251,24 @@ Launching the backup scheduled task
 Generating the public and private certificates
 ++++++++++++++++++++++++++++++++++++++++++++++
 
-* as root:
+* as root, generate an Ed25519 SSH key (modern and secure algorithm):
 
   .. code-block:: bash
 
-    ssh-keygen -t rsa -b 2048
+    ssh-keygen -t ed25519 -C "tisbackup@$(hostname)"
 
 * press :kbd:`Enter` for each one of the steps;
 
+.. note::
+
+  TISBackup supports Ed25519, ECDSA, and RSA key algorithms (in order of preference).
+  DSA keys are no longer supported for security reasons. If you need RSA for compatibility,
+  use at least 4096 bits:
+
+  .. code-block:: bash
+
+    ssh-keygen -t rsa -b 4096 -C "tisbackup@$(hostname)"
+
 |clap| You may now go on to the next step
 and :ref:`configure the backup jobs for your TISBackup<configuring_backup_jobs>`.