While I'm not a fan of excessive comments in code, I am a firm believer in keeping project documentation next to the project. I can't tell you how many places I've been where project documentation was spread across OneNote, Confluence, documents in SharePoint, etc, etc, etc. Any project I have this sort of control over will have the documentation for the project in the repo. I find having a git history of your documentation that can be viewed alongside the history for your code adds so much value to the documentation.
Sure, but throw it in a README.md, not in the code itself.
Besides, if you foster a culture of writing "why" docs, the problem of where to place these docs is one you solved early on, so people will know where to go.
Absolutely. If you were to survey, I would fall much more closely on the "don't comment code" side of things. But if you have documentation for your project, keep it close to the project. That's all.
