Migrate content out of confluence for how to contribute.

[epugh]

https://issues.apache.org/jira/browse/SOLR-16544

Description

Fix a link

Solution

Fix the link

Tests

Manually test

Checklist

Please review the following and check all that apply:

• 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)
• 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

[epugh]

Hey @janhoy actually I am using this branch to migrate the Wiki content over so we can eliminate the old HowToContribute ;-)

[epugh]

Need to delete https://cwiki.apache.org/confluence/display/SOLR/Git+commit+process and https://cwiki.apache.org/confluence/display/SOLR/HowToContribute when merged ;-) Or maybe, because I think https://cwiki.apache.org/confluence/display/SOLR/HowToContribute has been aroudn a long time, have a pointer to the CONTRIBUTING.md file...?