Add credential manager setup instructions

I’d like to add a page that outlines how to set up Git Credential Manager for use in different environments.

### macOS

```sh
git config --global credential.helper osxkeychain
```

### Windows

```sh
git config --global credential.helper wincred
```

### Windows Subsystem for Linux (WSL)

To set up Git Credential Manager for use with a WSL distribution, open your distribution and enter this command:

```sh
git config --global credential.helper "/mnt/c/Program\ Files/Git/mingw64/libexec/git-core/git-credential-manager.exe"
```