Ref Guide: fix wrapping & appearance of page nav buttons

The buttons at the bottom of each page of the Ref Guide (that allow the user to nav to the next or previous pages) are rather large and really orange. I think we could do better with a more subtle approach.

Additionally, if the next page has a long title, the wrapping of the footer is a mess. See https://lucene.apache.org/solr/guide/7_4/uploading-data-with-solr-cell-using-apache-tika.html for an example.