How to setup sitewide keyword search?
You will need Search API and Search API Database for this - though I suggest setting up Apache Solr + Search API Solr instead of Search API Database.


Search API
Search API Database


and/or also:


Search API Solr


Search API provides a lot more flexibility in terms of the types of search you want to configure and setup. Some functionality within say, Solr, allows for defining synonyms and abbreviations, as well as open you up to having facet based searches.