[bvey]

Documents the accounting exports of thelia/thelia#4310: the sales journal written as a FEC file and the tax summary.

This takes over #74 from @ThomasDaSilva. His four commits are kept as they are, replayed on main, with one commit on top for what the core pull request added since:

• the pages are dated 3.4;
• the optional account of commercial credit notes (709 in the French chart);
• the credit notes of the CreditNote module, booked as reversed pieces by rate;
• the warning on discounted orders that mix rates (the invoice itself collects the tax at the average of the rates);
• the report shown on the export job screen;
• the FEC file is written without the spreadsheet formula guard, and why;
• how long the files are kept (file deleted after one day, job and report after seven) and what the anonymisation of a customer changes in a later export.

Companion pull requests, to merge in this order:

Order	Pull request	Content
1	thelia/thelia#4310	journal, tax summary, FEC serializer, settings, report
2	thelia-templates/default-twig#207	chart of accounts screen, report of the export job
3	this one	documentation