Jinja: Switches to jinjanat

Switches the templating engine and pins Python to 3.13. No functional changes.

Pin Python version to 3.11

Unfortunately python 3.12 drops the module imp which is needed by j2cli.
Update j2cli to do not use the imp module seems to be non-trivial (see kolypto/j2cli#80 for more details)

So the simplest fix would be to pin the python version to 3.11

Fixes Docker base image issues

Due to a change to some packages the Docker image could no longer use the binary releases of the installed Python packages and tried to compile them itself. The compilation failed.

This release fixes the problem by changing the used base image to a non-Alpine version.

Update to new YAML syntax

Changes action and samples to the new action YAML syntax, updates documentation.

Reformatted repo for GitHub Marketplace

This version is correctly recognized as an action for publishing on the Marketplace.

The uses command has changed to cstuder/apprise-ga@master or cstuder/apprise-ga@v2.0.0.

Initial release

Working release including templating.