Add internal docs for CI minute provisioning

What does this MR do?

As part of customers-gitlab-com#2924 (closed), we are adding a new mechanism to store CI Minutes purchases.

This MR is one small part of that process:

• adding a new table / class (!62393 (merged))
• add an entity to represent the new model in the REST API (!63651 (merged))
• add an API endpoint for creating new records (!63853 (merged))
• add internal documentation 👈🏽 we are here

This work is all part of a larger proposal documented, here: https://gitlab.com/gitlab-org/customers-gitlab-com/blob/staging/doc/ci_minutes/provisioning_proposal.md

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.