Incorporating setup documentation for local environment

Encountered a few setup issues that took a while to dig through to figure out while working on an issue with CATcher. I think it would benefit first time contributors if the documentation covered the steps to set up the local dev environment were more detailed.

Here's a sample of the new documentation. As of this moment, I'm not entirely sure how to override the default `environment.gen.ts` with local fields (i.e. step 5) so would love to understand the team's stance on this as I was modifying the fields directly while leaving the file changes uncommitted.

```markdown
## Creating your local development environment

1. Set up [Gatekeeper](https://github.com/CATcher-org/gatekeeper), a local server used to enable
client-side applications to perform OAuth with GitHub. Follow the instructions on the repository.
2. Create a new GitHub organization to store all your repositories. More details can be found
[here.](../ug/instructors.md)
3. Create a new GitHub OAuth application under the new GitHub organization. More details can be
found [here.](https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/creating-an-oauth-app)

Set the homepage URL to your organization's
[default homepage URL.](https://docs.github.com/en/pages/getting-started-with-github-pages/about-github-pages#types-of-github-pages-sites)

Set the authorization callback's URL to `http://localhost:4200`.

4. Start Gatekeeper with `npm run start` and take note of the local address of the server.

5. Update `environment.gen.ts` with the appropriate base configuration such as:
- `accessTokenUrl`: local Gatekeeper server address
- `clientDataUrl`: `profiles.json` of your local `client_data` folder (see step (2) for more
information)
- `clientId`: client id of your GitHub app

6. Start CATcher locally with `npm run ng:serve:web`, select a profile, and login to grant CATcher
access to your GitHub account to begin.
```

Start by reviewing the proposed setup steps and the existing documentation in this repository, then verify the local Gatekeeper, GitHub OAuth, client_data, and environment.gen.ts workflow. Done means contributors can follow the local environment setup without guessing, including the team's guidance for local environment values and whether generated files should remain uncommitted.

Written by the indexing model from the issue text.