IA-3431 Document the rules implemented in the vaccine stocks module

[mathvdh]

Explain what problem this PR is resolving

Related JIRA tickets : IA-XXX, WC2-XXX, POLIO-XXX

Self proofreading checklist

• Did I use eslint and black formatters
• Is my code clear enough and well documented
• Are my typescript files well typed
• New translations have been added or updated if new strings have been introduced in the frontend
• My migrations file are included
• Are there enough tests
• Documentation has been included (for new feature)

Doc

• Tell us where the doc can be found (docs folder, wiki, in the code...)

Changes

Explain the changes that were made. The idea is not to list exhaustively all the changes made (GitHub already provides a full diff), but to help the reviewers better understand:

• which specific file changes go together, e.g: when creating a table in the front-end, there usually is a config file that goes with it
• the reasoning behind some changes, e.g: deleted files because they are now redundant
• the behaviour to expect, e.g: tooltip has purple background color because the client likes it so, changed a key in the API response to be consistent with other endpoints

How to test

Explain how to test your PR.
If a specific config is required explain it here (dataset, account, profile, ...)

Print screen / video

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Notes

Things that the reviewers should know: known bugs that are out of the scope of the PR, other trade-offs that were made.
If the PR depends on a PR in bluesquare-components, or merges into another PR (i.o. main), it should also be mentioned here

[quang-le]

I think this doc would fit better in the polio-doc repo: https://github.com/BLSQ/polio-doc