Align Contributors' Guides

[Loquacity]

Describe the bug
We have several locations with Contributors' Guides, let's align them in the correct place:

• .github directory: This is the correct place, so that the repo shows that we have a Contributors' Guide in the Insights tab. However, doing a PR to this directory is a little tricky.
• Templates repo CONTRIBUTING.md: This has the right filename, but is only a stub.
• Incubator repo base-template/contributors-guide.md: This has content, but is in the wrong location.

To Reproduce
Steps to reproduce the behavior:

1. Go to different locations in the project
2. See different contributors' Guides
3. Be confused

Expected behavior
A single guide with comprehensive information, in the .github directory, and available in the GH Insights tab.

[barbaricyawps]

Thanks for being willing to take this on, @Loquacity . This issue is related to a task needed for the website: thegooddocsproject/website-hugo#22

Would you be willing to let me know what you consolidate so that we can get them up on the website?