Including documentation for setting up GitHub application and Gatekeeper

[woojiahao]

Is your feature request related to a problem? Please describe.

The current CATcher documentation is relatively sparse on how to setup the full application for dev mode. It took me a while to realize what was all required.

Describe the solution you'd like

The key points to note are:

1. Setting up a GitHub application with a custom client id
2. Setting up Gatekeeper locally
3. Modifying/adding an environment file with the custom client id, gatekeeper redirect URL, and client data URL

Describe alternatives you've considered

Alternatively, dockerize both CATcher and Gatekeeper to launch on 2 separate ports but so they are all bundled for dev purposes. This is mostly because the environment is hardcoded to a few values which may not be entirely accurate for dev.

Additional context

N/A

[woojiahao]

I will be working on creating a PR for this on the documentation repository instead