Clarify documentation for community collection testing

[oraNod]

Docs for testing community collections is unclear, fragmented, and out of date. Purpose of this task is to work with the community folks to clarify things and ensure documentation supports community collection maintainers.

https://docs.ansible.com/ansible/latest/dev_guide/testing_units_modules.html#traps-for-maintaining-python-2-compatibility

Related stuff:

ansible-community/community-topics#156
Integration testing quick-start guide
Unit testing quick-start guide
Local PR testing guide
Maintainer guidelines does not contain any info about testing and CI ! After the testing docs are updated/created/re-arranged, we should add links to the guidelines.