Allow creation and deletion of generic files

[oliverjwroberts]

Description of change

Following on from #5989, this PR adds functionality to create and delete generic documents, along with their underlying specific-type document.

Test Instructions

1. Create generic documents via POST request to v4/document/ with the following payload:

{
    "document_type": "documents.sharepointdocument",
    "document_data": {
        "title": "Project Proposal",
        "url": "https://sharepoint.example.com/sites/project/proposal.docx"
    },
    "related_object_type": "company.company",
    "related_object_id": "<uuid of company>"
}

This example is specifically for a company sharepoint document, which is currently the only supported type.

2. Soft-delete (i.e. archive) generic documents with a DELETE request to v4/document/<uuid:pk>.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov]

Codecov Report

Attention: Patch coverage is 98.43750% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.66%. Comparing base (68c27c0) to head (d4fc667).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
datahub/documents/views.py	97.05%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5990      +/-   ##
==========================================
- Coverage   96.67%   96.66%   -0.01%     
==========================================
  Files        1092     1092              
  Lines       25702    25759      +57     
  Branches     1694     1697       +3     
==========================================
+ Hits        24847    24900      +53     
- Misses        698      700       +2     
- Partials      157      159       +2     

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[code-review-doctor]

Some food for thought. View full project report here.