Improve documentation on "How to Contribute"

Currently the main location for information on how to contribute to the Solr open source project is here: https://cwiki.apache.org/confluence/display/solr/HowToContribute. As colleagues I work with are increasing our contributions to the Solr community, we found this documentation difficult to parse and it seems like it could use some love.

My idea is to write up the current best practices for contributing within the dev-docs README in Github: https://github.com/apache/solr/blob/main/dev-docs/README.adoc and link to that from the Solr reference docs to make it more easily findable. This will provide the information for how to contribute in a version controlled location, making updates easier in the future as well.