Added "Solidity" to the table of contents. (#6751)

* Added Solidity to table of contents

* Added Solidity to the table of contents and moved solidity section from the java section.

* Solidity to heading level 3

Co-authored-by: CleanMachine1 <78213164+CleanMachine1@users.noreply.github.com>

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: CleanMachine1 <78213164+CleanMachine1@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -195,6 +195,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Self](#self)
 * [Smalltalk](#smalltalk)
 * [Snap](#snap)
+* [Solidity](#solidity)
 * [Spark](#spark)
 * [Splunk](#splunk)
 * [SQL (implementation agnostic)](#sql-implementation-agnostic)
@@ -1074,12 +1075,6 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [The JasperReports Ultimate Guide, Third Edition](http://jasperreports.sourceforge.net/JasperReports-Ultimate-Guide-3.pdf) (PDF)
 
 
-#### Solidity
-
-* [Introductory guide for Solidity](https://www.tutorialspoint.com/solidity/index.htm)
-* [The Solidity Reference Guide](https://docs.soliditylang.org)
-
-
 #### Spring
 
 * [Building Applications with Spring 5 and Vue.js 2](https://www.packtpub.com/free-ebooks/building-applications-spring-5-and-vuejs-2) - James J. Ye (Packt account *required*)
@@ -2279,6 +2274,12 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Snap! Reference Manual](https://snap.berkeley.edu/snapsource/help/SnapManual.pdf) - B. Harvey, J. Mönig (PDF)
 
 
+### Solidity
+
+* [Introductory guide for Solidity](https://www.tutorialspoint.com/solidity/index.htm)
+* [The Solidity Reference Guide](https://docs.soliditylang.org)
+
+
 ### Spark
 
 * [Databricks Spark Knowledge Base](https://www.gitbook.com/book/databricks/databricks-spark-knowledge-base/details)