[DOC] External OAUTH2 App dev environment instructions

[fearnworks]

Describe the issue

Add SOP style instructions in ./doc/contributing/environment_setup outlining how to set up test applications for github and discord authentication and to verify proper setup. This should cover the environment values for

GITHUB_CLIENT_ID=github_client_id
GITHUB_CLIENT_SECRET=github_client_secret

DISCORD_CLIENT_ID=discord_client_id
DISCORD_CLIENT_SECRET=discord_client_secret

If possible try and include some sort of visual guidance as well.

Location of the issue

./doc/contributing/environment_setup
.env.template
Provide a URL or file path where the issue occurs.

Acceptance Criteria:

• Tested with feedback from someone who has not done this setup already
• Github Auth Success
• Discord Auth Success