Document Solr plugin configuration with TYPO3 access filter

[catplat]

Priority

3 - Not urgent - can be addressed as team bandwidth permits

Where should the changes be made?

https://docs.upsun.com/add-services/solr.html
https://fixed.docs.upsun.com/add-services/solr.html

What exactly should be updated?

Document how to load a custom TYPO3 Solr plugin JAR in a Solr 9.x service on Upsun.

Optional: Additional context

Blocked by Linear IMB-3488