[Support Docs] Update Support Docs with Better & Clearer Pytest.ini info. #2953

Per issues #2536 , #2539 and #2745 -- Without a pytest.ini file placed at the root of the exercise repo, Pytest will throw "unregistered mark" warnings for the marked test tasks in our test files.

Since we have not resolved having this file download automatically with the CLI, we need to be very clear in our docs with how student get these warnings to disappear/silence. We should include:

• How to run silenced from the command line
• How to set an env var for running silent/registering marks
• How to place a pytest.ini file to register the marks
• How to create a pytest.ini file from scratch to register marks

Documents to review and update:

• python/exercises/shared/.docs/HELP.md (this is downloaded via CLI)
• python/docs/TESTS.md (this is only available on website, consider a copy for download via CLI)
• python/docs/TOOLS.md (this is only available on the website, consider a copy for download via CLI)
• python/docs/RESOURCES.md (website) (to add docs on pytest and other tools we use)