Improve docs on how to contribute and fix hanging test issues.

Changelogs

Fix #72 with additional description on contribute.md for pip-only users
Add ruff for linting
Fix #74 by allowing users to ignore descriptors for RDKitDescriptors2D. Also remove test that could hang forever

Checklist:

[x] Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
[x] Add tests to cover the fixed bug(s) or the new introduced feature(s) (if appropriate).
[x] Update the API documentation if a new function is added, or an existing one is deleted. Eventually consider making a new tutorial for new features.
[x] Write concise and explanatory changelogs below.
[x] If possible, assign one of the following labels to the PR: feature, fix or test (or ask a maintainer to do it for you).

This PR add ruff as linter and address contribution guideline for pip users.