Add contributor documentation for how to run header/reST checks, codespell and other linting locally #2265

Right now, we run some basic pre-commit checks of the headers and reST on the CIs, as added in #1802 and extended in #1886 (based off #1803 ), which were also intended to allow PEP authors/editors to check their PEPs locally. However, the readme, contributing guide, nor /docs explain how to do this, or even explicitly mention it. Therefore, I'll add a step by step explanation to the contributing guide, along with some form of the admonition mentioned in #2151 .