Renamed doc_collections to just-the-docs.collections

Improved search documentation

duplicated from fix #80 
the same error in Version 2.0.7
fix with same procedure

Because `page.title` (and `page.url`) is already escaped and page titles served by lunrjs do not need escaping, ampersands need to be unescaped to display properly in search results.

and mention `search_exclude`