Update wiki documentation for github enterprise, add missing workflow scope

[ThomasBachSap]

There is some github enterprise documentation in the wiki: https://github.com/microsoft/vscode-pull-request-github/wiki#2-is-github-enterprise-supported However, it does not contain the correct list of scopes for the personal access token as ´workflow´ is also required. In addition, some parts seem to be a bit outdated.

I have drafted an update here: https://github.com/ThomasBachSap/vscode-pull-request-github/wiki/Home/_compare/5fe3dbb169ebf7e9b957fb80c8fd90f8dd516f9c...d4272a1a55e3f487bf5ad20d03201cc2edda7c18?diff=split

Unfortunately, I do not see an option to create a pull request for the wiki. Happy to create if somebody points me to some info how to do it. Otherwise, one just might copy&paste an edit. Consider the text released as CC0 where possible and CC-BY-4.0 otherwise.