Add paragraph about GitHub 2FA and scc commands

- Add link on how to create Tokens using GitHub interface
- Add command to set personal access token globally
- Mention connection via username/password is not possible if 2FA is on
Typo fix
Fix link to 2FA on help.github.com
Use http rather than https links for Sphinx linkchecker
Merge pull request #565 from sbesson/11760_scc_github2fa

Add paragraph about GitHub 2FA and scc commands