checkJavadocLinks.py doesn't allow links to new Ref Guide in javadocs

In SOLR-11135 I'm fixing a number of URLs in source that point to the old Solr Reference Guide location (https://cwiki.apache.org/confluence/display/solr/...). The new base URL for the Ref Guide is https://lucene.apache.org/solr/guide... which is the same as the javadocs.

Several of these references are in Java classes, but changing those to the new URLs causes precommit to fail because checkJavadocLinks.py doesn't allow links in javadocs to contain URLs starting with lucene.apache.org unless they are explicitly allowed.

Fixing this may not be as simple as just allowing any URL starting with https://lucene.apache.org/solr/guide.... For javadocs we want to only use non-versioned urls, but someone could accidentally insert a versioned URL (say, for 7.0) that would be invalid in later releases.

Since javadocs & ref guide are on the same server, perhaps some sort of relative link is preferable, but I honestly don't know enough about URL construction in javadocs to know what sorts of options are available.