Improve example_gallery script

Changes:
- Fix unnecessary ../ in some file names of certain examples which have
the .pyproject file in the doc directory. e.g. widget binding example
- Add .html code highlighting to the code snippets
- Fix file docstring (__doc__)

Refactoring:
- Add type hints to functions
- General code clean up

Change-Id: Iaebabf10f59aa515613566256a94bece65b4ffd3
Reviewed-by: Friedemann Kleint <[email protected]>
Reviewed-by: Shyamnath Premnadh <[email protected]>

Author: Jaime Resano