Y25-708 - Document current accessioning process

[StephenHulme]

Closes #5419

Compare to before the accessioning refactor started in #5319

Changes proposed in this pull request

• Add mermaid chart of accessioning call-stack

WIP:

Further WIP:

Instructions for Reviewers

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
    - Check story numbers included
    - Check for debug code
    - Check version

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.20%. Comparing base (90e3a01) to head (dee805b).
⚠️ Report is 43 commits behind head on y25-709-remove-obsolete-accessioning-code.

Additional details and impacted files

@@                              Coverage Diff                              @@
##           y25-709-remove-obsolete-accessioning-code    #5504      +/-   ##
=============================================================================
- Coverage                                      87.23%   87.20%   -0.03%     
=============================================================================
  Files                                           1459     1460       +1     
  Lines                                          32851    32907      +56     
  Branches                                        3449     3460      +11     
=============================================================================
+ Hits                                           28656    28696      +40     
- Misses                                          4174     4190      +16     
  Partials                                          21       21              

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