Include Elasticsearch study material (#8449)

* include Elasticsearch study material

* restore space between chapters

* remove elasticsearch notes from free-programming-books-langs.md file

books/free-programming-books-subjects.md

@@ -740,6 +740,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 ### Search Engines
 
+* [Elasticsearch Certified Engineer notes ](https://www.pistocop.dev/posts/es_engineer_exam_notes/) - (HTML)
 * [Elasticsearch: The Definitive Guide](https://www.elastic.co/guide/en/elasticsearch/guide/current/index.html) ([fork it on GH](https://github.com/elastic/elasticsearch-definitive-guide))
 * [Search Engines: Information Retrieval in Practice](https://ciir.cs.umass.edu/irbook) - W. Bruce Croft, Donald Metzler, Trevor Strohman (PDF)
 * [Solr for newbies workshop (2019)](https://github.com/hectorcorrea/solr-for-newbies) - Hector Correa ([PDF](https://github.com/hectorcorrea/solr-for-newbies/blob/master/tutorial.pdf))