Add Gremlin Section to Programming Books (#10712)

* Add Gremlin Section to Programming Books

* Update books/free-programming-books-langs.md

Co-authored-by: Eric Hellman <eric@hellman.net>

* Create Graph section to accomodate graph tecnologies

* Fix Wrong break lines

---------

Co-authored-by: Eric Hellman <eric@hellman.net>

books/free-programming-books-langs.md

@@ -64,7 +64,10 @@ Books on general-purpose programming that don't focus on a specific language are
 * [Fortran](#fortran)
 * [FreeBSD](#freebsd)
 * [Go](#go)
-* [GraphQL](#graphql)
+* [Graphs](#graphs)
+    * [GraphQL](#graphql)
+    * [Gremlin](#gremlin)
+    * [Neo4J](#neo4j)
 * [Groovy](#groovy)
     * [Gradle](#gradle)
     * [Grails](#grails)
@@ -139,7 +142,6 @@ Books on general-purpose programming that don't focus on a specific language are
 * [Modelica](#modelica)
 * [MongoDB](#mongodb)
 * [MySQL](#mysql)
-* [Neo4J](#neo4j)
 * [.NET Core / .NET](#net-core)
 * [.NET Framework](#net-framework)
 * [NewSQL](#newsql)
@@ -842,13 +844,29 @@ Books on general-purpose programming that don't focus on a specific language are
 * [Web apps in Go, the anti textbook](https://github.com/thewhitetulip/web-dev-golang-anti-textbook/)
 
 
-### GraphQL
+### Graphs
+
+#### GraphQL
 
 * [Fullstack GraphQL](https://github.com/GraphQLCollege/fullstack-graphql)
 * [GraphQL or Bust](https://nordicapis.com/wp-content/uploads/GraphQL-or-Bust-v2.2.pdf) - Nordic APIs (PDF)
 * [Learning graphqL](https://riptutorial.com/Download/graphql.pdf) - Compiled from StackOverflow Documentation (PDF)
 
 
+#### Gremlin
+
+* [Practical Gremlin - An Apache TinkerPop Tutorial](https://www.kelvinlawrence.net/book/PracticalGremlin.html) - Kelvin R. Lawrence
+
+
+#### Neo4J
+
+* [Fullstack GraphQL Applications with GRANDStack – Essential Excerpts](https://neo4j.com/fullstack-graphql-applications-with-grandstack/) - William Lyon (PDF) *(email requested)*
+* [Graph Algorithms: Practical Examples in Apache Spark and Neo4j](https://neo4j.com/graph-algorithms-book/) - Mark Needham, Amy E. Hodler (PDF, EPUB, MOBI) *(email requested)*
+* [Graph Databases 2nd edition](http://neo4j.com/books/graph-databases/) - Ian Robinson, Jim Webber, Emil Eifrém (PDF, EPUB, MOBI) *(email requested)*
+* [Graph Databases For Dummies](https://neo4j.com/graph-databases-for-dummies/) - Dr. Jim Webber, Rik Van Bruggen (PDF) *(email requested)*
+* [Knowledge Graphs: Data in Context for Responsive Businesses](https://neo4j.com/knowledge-graphs-data-in-context-for-responsive-businesses/) - Jesús Barrasa, Amy E. Hodler, Jim Webber (PDF) *(email requested)*
+
+
 ### Groovy
 
 #### Gradle
@@ -1622,15 +1640,6 @@ Books on general-purpose programming that don't focus on a specific language are
 * [MySQL Notes for Professionals](https://goalkicker.com/MySQLBook/) - Compiled from StackOverflow Documentation (PDF)
 
 
-### Neo4J
-
-* [Fullstack GraphQL Applications with GRANDStack – Essential Excerpts](https://neo4j.com/fullstack-graphql-applications-with-grandstack/) - William Lyon (PDF) *(email requested)*
-* [Graph Algorithms: Practical Examples in Apache Spark and Neo4j](https://neo4j.com/graph-algorithms-book/) - Mark Needham, Amy E. Hodler (PDF, EPUB, MOBI) *(email requested)*
-* [Graph Databases 2nd edition](http://neo4j.com/books/graph-databases/) - Ian Robinson, Jim Webber, Emil Eifrém (PDF, EPUB, MOBI) *(email requested)*
-* [Graph Databases For Dummies](https://neo4j.com/graph-databases-for-dummies/) - Dr. Jim Webber, Rik Van Bruggen (PDF) *(email requested)*
-* [Knowledge Graphs: Data in Context for Responsive Businesses](https://neo4j.com/knowledge-graphs-data-in-context-for-responsive-businesses/) - Jesús Barrasa, Amy E. Hodler, Jim Webber (PDF) *(email requested)*
-
-
 ### .NET Core
 
 * [Clean Code .NET](https://github.com/thangchung/clean-code-dotnet)