[docs] Reorganized docs files

[kagarmoe]

Description of changes:

Adds documentation under the /content directory. This is intended to give you a place to write that is separate from the existing documentation. Feel free to use a different approach.

The user guide hews closely to the API. Try to remember that you're explaining to users how to think about and get things done with s2n-tls. Focus on the user, not on the product.

The developer guide has some content that might be better in the user guide. I've left notes.

Move content around and experiment as you see fit. Good luck!

[github-actions[bot]]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dougch]

partially superseded by #4343