Simplifying CONTRIBUTING.md for new contributors.
Updating the "Contributing to the Codebase" section to make it more beginner friendly.  It is now in a bullet point format with links and commands needed to contribute.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

@jreback no worries.  I had to make a contribution to an OSS project for school.  I'm not familiar enough with Pandas codebase to contribute bug fixes, but perhaps one day no worries - have a look at the "good first issue" label if you want something accessible to get started with

If you're familiar with mypy / static typing, then there's lots of work to be done in that space

closing for now then