doc: add resources for variable and data types (#4948)

Add links for variables and data types from The Book (Official docs)

src/data/roadmaps/rust/content/101-language-basics/100-syntax/101-variables.md

@@ -1,3 +1,8 @@
 # Variables, Constants, and Data Types
 
 In Rust, variables are declared using the `let` keyword. They are immutable by default, which means once a value is bound to a variable, it cannot be changed. If you want to make a variable mutable, the `mut` keyword is used. So, if you wanted to declare a mutable variable `x` and assign it the value `5`, you would write `let mut x = 5;`. Variables can also be patterned. By default in Rust, variables are block-scoped. Rust also supports several types of variable attributes.
+
+Learn more from the following links:
+
+- [Variables and Mutability](https://rust-book.cs.brown.edu/ch03-01-variables-and-mutability.html)
+- [Data Types](https://rust-book.cs.brown.edu/ch03-02-data-types.html)