Refactor Versioning, documentation and CI workflows

[armando-palacio]

This pull request implements various enhancements and optimizations to the project setup and maintenance:

1. Refactored Versioning and Documentation Handling:

   • Introduced a __version__ variable in __init__.py.
   • Updated Python requirements to >3.8 in setup.py file.
   • Removed redundant titles from README.md.

2. Simplified Version Management:

   • Removed VERSION.txt, updating version directly in __init__.py.

3. Enhanced Documentation Handling:

   • Modified sphinx conf.py to retrieve version from opticmlib.

4. Code Cleanup:

   • Removed unnecessary imports and unused code.

5. Improved Setup.py Requirements Handling:

   • Updated setup.py to include requirements using include_package_data=True argument, along with the addition of a MANIFEST.in file.

6. Enhanced CI Process:

   • Added checks for unittests, package build, and sphinx docs to CI.
   • Integrated release and publish workflows into test-build-release-publish.yml.

7. Documentation Improvement:

   • Added docstrings to get_long_description and get_requirements functions in setup.py .