Generated documentation should link to the default codec's index format documentation, rather than being hard coded in lucene/site/xsl/index.xsl

In the 4.6 RC1, The "File Formats” link from the generated Lucene Documentation page leads to the 4.5 format doc, rather than the 4.6 format doc (Lucene46Codec was introduced by LUCENE-5215).

Updating this is not automated now - it’s hard-coded in lucene/site/xsl/index.xsl - the default codec doesn’t change in every release.

The default codec could be extracted from o.a.l.codecs.Codec#defaultCodec [ = Codec.forName(“Lucene46”)] and inserted into the URL to the index file format documentation.