Adding gidgethub to GitHub's documentation

[Mariatta]

I created a PR to add gidgethub to the GitHub's list of third party Python libraries github/docs#2226

[brettcannon]

😄 I wrote a note to myself this weekend to do this today. Thanks! Did their blog post on ghapi make you want to do this, too?

[Mariatta]

While working on #145 I came across the documentation, and just wondered "why isn't gidgethub on that list?" 😛

[Mariatta]

This is now live! 😎 https://docs.github.com/en/free-pro-team@latest/rest/overview/libraries#python