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:

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 (46597ff) to head (f13e13c).
⚠️ Report is 20 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                                      87.20%   87.20%           
==========================================================================
  Files                                           1455     1455           
  Lines                                          32598    32600    +2     
  Branches                                        3410     3410           
==========================================================================
+ Hits                                           28426    28430    +4     
+ Misses                                          4157     4155    -2     
  Partials                                          15       15           

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