Add more B2B documentation

[jkachel]

What are the relevant tickets?

n/a

Description (What does it do?)

Fills out the B2B system documentation:

• Adds more documentation for the management commands
• Adds a bunch of technical information about how the system is structured and how it operates
• Adds some detail for the user-facing interfaces and some run-downs of common scenarios

This also makes some minor changes to the Sphinx configuration to get MyST parser set up. This allows us to write docs in something mostly resembling Markdown along with reStructuredText.

How can this be tested?

Run pants docs :: to generate the docs, then read them. They'll be in dist/sphinx/index.html.

The docs should make sense and cover most everything. Some things are not fully documented on purpose - the management commands, for one; they're covered but the detail is left to the help in the command itself.

[github-actions]

OpenAPI Changes

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