SOLR-17583: Bring back documentation for Adding Custom Expressions

[cfeldmann]

https://issues.apache.org/jira/browse/SOLR-17583

Description

The section for adding Custom Expressions was removed in Solr 8.8. This change brings it back.

Solution

This change was cherry-picked from the same change on the main branch.

Tests

I ran ./gradlew tidy updateLicenses check -x test and then opened my local copy of the Reference Guide. I verified that I see the new text and that the link to the Expressible javadoc is working.

Checklist

Please review the following and check all that apply:

• [ x] I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• [ x] I have created a Jira issue and added the issue ID to my pull request title.
• [ x] I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• [ x] I have run ./gradlew check.
• I have added tests for my changes.
• [ x] I have added documentation for the Reference Guide