Add an MCP endpoint to search the Solr ref guide by version

Add an MCP endpoint to search the Solr ref guide by version. For example, the user should be able to request "circuit breakers" and Solr 9.10 and return the URLs of relevant ref guide pages.

Note that the MCP endpoint doesn't return blocks of text, but instead it returns URLs of relevant Solr ref guide pages.

Please make sure that approach sounds ok and feel free to assign to me and I'll make a PR.

The issue names no files or tests. Start by inspecting the existing MCP endpoints and the Solr reference-guide search path, then determine how a query and Solr version are passed. Done means a request such as “circuit breakers” with Solr 9.10 returns relevant reference-guide URLs rather than blocks of text.

Written by the indexing model from the issue text.