Clean up CONTRIBUTING.md docs and change _is_named_isotope getattr default value

[DanielYang59]

• [I believe this would not be breaking] The default value as False over None might be more conventional (we should not use truthy to check None):

  # When the `_is_named_isotope` doesn't have `_is_named_isotope`,
  # we're effectively checking `if None`
  - if getattr(obj, "_is_named_isotope", None):
  + if getattr(obj, "_is_named_isotope", False):

• Recommend Google Style Docstring in CONTRIBUTING.md
• Clarify usage of PMG_TEST_FILES_DIR when running unit test
• Fix internal indexes

[DanielYang59]

I think it might be good to provide this alternative as I think most new contributors don't really need the complete commit history or other branches, they would just branch from main.

To clone the complete repo:

git clone https://github.com/materialsproject/pymatgen.git  48.11s user 15.68s system 69% cpu 1:31.62 total

Depth one clone:

git clone https://github.com/materialsproject/pymatgen.git --depth 1  7.60s user 2.75s system 31% cpu 32.622 total

[DanielYang59]

I believe setting PMG_TEST_FILES_DIR is not needed for editable mode, however we might still leave the "non-editable install + set PMG_TEST_FILES_DIR" as an option here, as there might be users who want to install the package in non-editable mode but still run tests, say people who don't intend to develop pymatgen and just run tests to verify pymatgen integration.