Add TTS org to "Intro to GitHub"

[astrogeco]

Pages

• https://handbook.tts.gsa.gov/training-and-development/intro-to-github/
• https://handbook.tts.gsa.gov/tools/github/

Background Information

The intro to github and github pages are missing the new TTS org.

Acceptance Criteria

• [ ] Add a section with information on the TTS org
• [ ] Add guidance on when to use 18F vs TTS
• [ ] Add instructions on requesting access to TTS org