Document how to use multiple GitHub accounts with Copilot better

[TylerLeonhardt]

Follow up from microsoft/vscode#276028

There are two parts to this:

1. For GitHub.com accounts, call out that the user should use Manage Extension Account preference to sign in to other accounts... this preference is saved per-workspace/per-profile so you're able to have different workspaces with different preferences.
2. For ghe.com accounts, it's more complicated... you have the VS Code setting:

   "github.copilot.advanced": { "authProvider": "github-enterprise" }
   

   which can be set per-profile or per-workspace

So, it's possible to be signed in to a GHE account for Copilot, and a GitHub.com account for GH PR extension... and vice versa...OR 1 GitHub.com account for Copilot and a different GitHub.com account for GH PR but it's maybe not so obvious.

[ntrogh]

@TylerLeonhardt I've updated the docs accordingly. Can you review to make sure the steps are correct?