[Doc]: consolidate coding guidelines  #26095

Documentation Link

https://matplotlib.org/devdocs/devel/contributing.html#coding-guidelines

Problem

There is both a coding guidelines section in contribute and a whole separate page of coding guidelines that used to be a pull request guide:

• https://matplotlib.org/devdocs/devel/contributing.html#coding-guidelines
• https://matplotlib.org/devdocs/devel/coding_guide.html

They're kind of repeating and referencing each other in a way where contributors should probably be reading both but they're not really cross linked. For example, the adding new API section of each:

Suggested improvement

I think the coding guidelines section of the contributing guide should be merged into the coding_guide page and I'd pull the PR checklists back out into their own page. I'd rather merge than cross link because I think "one source of truth"/"one place to update" is more maintainable than trying to keep all the links in sync.