Migrate repo wiki guide to CONTRIBUTING.md

[mao-sz]

Because

Repo wikis are awkward. Only editable by org members, no PRs and no reviews for changes. It's also tied to GitHub, not the repo itself, so can only be viewed in a separate window on GitHub.

We don't need so many pages to warrant the repo wiki. We can do away with the FAQs, contributing guide, testing etc. and only really need the setup instructions, and instructions for adding migrations/slash commands.

This can all be put in a contributing guide file.

This PR

• Creates CONTRIBUTING.md for setup, migrations, and slash command info.

Issue

Closes #861

Additional Information

After merging, I'll purge and disable the repo wiki

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. Callbacks command: Update verbiage
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR adds new features or functionality, I have added new tests
• If applicable, I have ensured all tests related to any command files included in this PR pass, and/or all snapshots are up to date