added maths section and numberous courses underit, different headings for different topics in free-courses-hi.md (#7312)

* added mathematics and two subheadings-discreteMaths and Stats and probability

* Update free-courses-hi.md

* added mathematics and two subheadings-discrete maths and probability

* removed two subheadings and stacked all courses under one

* Update free-courses-hi.md

* Update courses/free-courses-hi.md

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

* Update courses/free-courses-hi.md

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

* Update courses/free-courses-hi.md

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

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

courses/free-courses-hi.md

@@ -26,6 +26,7 @@
 * [Kotlin](#kotlin)
 * [Linux](#linux)
 * [Machine Learning](#machine-learning)
+* [Mathematics](#mathematics)
 * [PHP](#php)
 * [Python](#python)
     * [Django](#django)
@@ -251,6 +252,13 @@
 * [Machine Learning Tutorials For Beginners Using Python in Hindi](https://www.youtube.com/playlist?list=PLu0W_9lII9ai6fAMHp-acBmJONT7Y4BSG) - CodeWithHarry (YouTube)
 
 
+### Mathematics
+
+* [Discrete Mathematics](https://www.youtube.com/playlist?list=PLxCzCOWd7aiH2wwES9vPWsEL6ipTaUSl3) - Gate Smashers, `tch.:` Varun Singla, `tch.:` Naina Wadhwa Singla
+* [Discrete Mathematics (Full Course) By Dr.Gajendra Purohit](https://www.youtube.com/playlist?list=PLU6SqdYcYsfJ27O0dvuMwafS3X8CecqUg) - Gajendra Purohit
+* [Statistics And Probability](https://www.youtube.com/playlist?list=PLU6SqdYcYsfLRq3tu-g_hvkHDcorrtcBK) - Gajendra Purohit
+
+
 ### PHP
 
 * [Core PHP (Hindi)](https://www.youtube.com/playlist?list=PLbGui_ZYuhigFdLdbSI2EM2MrJB7I0j-B) - Rajesh Kumar, Geeky Shows