docs: Rewrite README, add InvoiceSalesOrderResult::findByProcessId()

[stephangroen]

Summary

README rewrite

The README is restructured around what users need to do, and brought up to date with the current library.

Examples that led to broken setups are fixed:

• Tokens were only stored after connect(). Refresh tokens rotate, so a refresh during later requests lost the new token and locked users out. The README now puts setTokenUpdateCallback() front and centre.
• The lock and unlock callbacks were set after connect(), so they weren't active during the first refresh. The multi-process locking pattern is now explained, including setRefreshAccessTokenCallback().
• $item->find($id); and $item->get(); threw away their results.
• download() was documented as returning a string. It returns a stream.
• Tokens were run through serialize()/unserialize() for no reason.

Newly documented: error handling and status codes; all filter() options ($expand, $select, other OData options, headers such as CustomDescriptionLanguage); first() and findId(); manual paging with getResultSet(); sync endpoints (Timestamp) and bulk endpoints; lazy loading versus $expand; why to always set the division; base urls per country; state for CSRF; logging via middleware; webhook code examples; the daily maintenance window (#691).

Also: a table of contents, badges, updated troubleshooting, a versions table with upgrade notes for 4.7 and 4.8, and a contributing section that links the entity generator. The job openings link and text copied from an old PR are removed. The #invoice-sales-orders-async anchor (linked from the v4.9.0 release notes) still works.

InvoiceSalesOrderResult::findByProcessId()

$result = (new InvoiceSalesOrderResult($connection))->findByProcessId($processId);

This keeps the ProcessID parameter format (guid'…') inside the library. If the format turns out to be wrong, a patch release fixes it for everyone, without users changing their code. get(['ProcessID' => …]) keeps working.

Test plan

• All 27 PHP snippets in the README pass php -l
• Every field, method and class used in the README exists in the library
• 2 new tests for findByProcessId(); vendor/bin/phpunit (598 tests) and vendor/bin/phpstan pass
• CI passes

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