Documentation & License Updates

[lrljoe]

This PR performs the following changes:

1. Changes use of gmail to the rappasoft email in license.md
2. Changes "I" to "we" in introduction.md
3. Removes Slack reference, adds reference to the Discord discussion post, so that the Discord link is always accurate
4. Adds documentation on Tailwind Dark Mode configuration
5. Adds reference to benefits for Sponsorship.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests and did you add any new tests needed for your feature?
2. Did you update all templates (if applicable)?
3. Did you add the relevant documentation (if applicable)?
4. Did you test locally to make sure your feature works as intended?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?