Please note that this PR targets the new_web_theme branch!

Towards #28084. This uses jinja2 to convert template rst into actual rst. My reasons are:

• It simplifies conf.py. Instead of creating a different function to generate each file, the templating approach allows to generate all files with a uniform function.
• It is easier to know the structure from a template rst instead of by looking at the generating function.
• One would have syntax highlighting (to some extent) for those files.
• I found pandas using this approach and thought it is nice.

By the way I forgot to add doc/index.rst to .gitignore in #28331 and added it here.

• Index page
• Minimum versions table (scroll down a bit)