Add guides for new complex contributions

[CarrotManMatt]

Add details in CONTRIBUTING.md on how to create a new command within a cog, and how to attach it to the bot

[CarrotManMatt]

Documentation for how to create a response button should also be added.

These docs can be added to a Guides subheading within CONTRIBUTING.md.

[CarrotManMatt]

Documentation for how to create a new environment variable should also be added.

[CarrotManMatt]

Documentation for how to create & interact with the Django models should also be added, along with information on the member retrieval DB queries via hashed Discord ID.

[CarrotManMatt]

I think it would also be beneficial to mention that changes involving GDPR sensitive data (E.g. handling of student IDs, member strikes, hashing of IDs & personal data) must be discussed and have a majority approval by the UoB CSS committee.