Improve guidelines for applying to gsoc at Tor

Rust lang has a good guideline on helping candidates to apply: https://github.com/rust-lang/google-summer-of-code/blob/main/gsoc%2Fproposal-guide.md

At Tor we could:

• add code of conduct
• specify who are the org admins that can help in general with gsoc at Tor
• how to improve on chances of being approved (connect with mentor, etc)