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

I found the source for the Custom Expressions section in the 8.7 docs and copied It to the current streaming-expressions.adoc page. The Expressible interface has been moved into the solrj-streaming package since version 8.7 so I updated the link to this interface to point to the correct javadoc location.

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)
• [x ] 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

[mkhludnev]

Thanks for PR. Have you tried to generate the doc? I'm not sure if the reference syntax hasn't changed from those times, and link might be broken.

[mkhludnev]

Build it myself. Link to javadoc works fine. Merging. I don't think it deserves CHANGES row.

[mkhludnev]

@cfeldmann please cherry-pick this commin to https://github.com/apache/solr/tree/branch_9x and make a PR.
Thanks!

[janhoy]

It can even be cherry picked to branch_9_7 for immediate visibility in the 9.7 guide.

[cfeldmann]

Ok. I've opened PRs to cherry-pick this change to both branch_9x and branch_9_7:
#2904
#2905