SOLR-17398: Link V1 collection operations to their V2 equivalents

[serhiy-bzhezytskyy]

https://issues.apache.org/jira/browse/SOLR-17398

Description

The V1 collection operations that do not simply call a V2 class give no hint where the V2 equivalent is.

Solution

CollectionsHandler gets a Javadoc link to the V2 class, with its path and command, on the ten operations that have none. Two, REINDEXCOLLECTION and DISTRIBUTEDAPIPROCESSING, say there is no V2 equivalent yet. Javadoc only. Written with Claude Code.

Tests

./gradlew check -x test is clean (rat skipped); the links are resolved by the Javadoc build.

Checklist

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• 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.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide
• I have added a changelog entry for my change