Expand and refine Workbench documentation

[seansica]

Add documentation for each of the following:

User-centric:

• Add doc explaining how all posted content is validated by the ATT&CK Data Model
• Add doc explaining how the backend generates the ATT&CK ID and the conditions under which a new ID is generated (e.g., new object, technique to sub conversion & vice versa)

Admin-centric:

• Add doc explaining how to upgrade Workbench (related to migrations)
• Add doc explaining how the scheduler is configured and which scheduled tasks exist today

Developer-centric:

• Add doc explaining the data pipeline for create and updateFull. Clarify how the object is composed from the req.body and validated by the ADM right before passing the payload to the repository layer for persistence. Also clarify which keys are explicitly required versus optional and which keys are idempotent and under the control of the backend
• Add doc explaining the Mongoose models and how we explicitly avoid serializing keys to empty lists, instead defaulting to undefined with the explicit goal of having persisted documents be as STIX compliant as possible so as to minimize the amount of post-processing needed during STIX bundle export
• Add doc explaining how database migrations are applied, which migrations exist today, and why
• Add doc explaining how the scheduler works
• Add doc explaining how the CI/CD pipelines work
• Add doc explaining how semantic release is used
• Add doc explaining the Docker tagging strategy