Usage improvements for theme / template developers.

New features:

- Trace template usage if env variable NIKOLA_TEMPLATES_TRACE is set.
- Enable Jinja2 extensions via theme configuration.
- Clarify template documentation: A parent template is not strictly needed.

Additional code improvements
helpful for those developing templates who actually read the code:

- Added several typing hints and improved others.
- A few code comments were added or improved.