Add documentation on how the HTML reports are created

Description
This PR is used to track the progress of adding the documentation for maintaining the HTML reports created by Macaron (in the Sphinx documentation's Deverloper Reference section or as a README).
Some of contents that I currently think of (and potentially add more in the future):

• The extensions filters and tests defined in https://github.com/oracle/macaron/blob/main/src/macaron/output_reporter/jinja2_extensions.py:
  • How we can use it?
  • How it's passed into the Jinja2 environment
• How to render the tree-view structure in the HTML reports (i.e. how the recursive feature of for control structure in Jinja2 is utilized. how the CSS and Javascript in the HTML files are used, etc.)
• Inheritance and macros of Jinja2 templates here
• White-space control