docs: Add Documentation for Combinatorics and Permutations in C++

Author: LuchoBazz

Diff for: website/docs/math/combinatorics.md

@@ -27,6 +27,14 @@ int64_t fastpow(int64_t a, int64_t b) {
 ```
 Efficiently computes $a^b \mod \text{md}$ using recursive exponentiation by squaring. This function operates in $O(\log b)$ time, making it suitable for handling large exponents.
 
+#### `inverse`  
+```cpp
+auto inverse = [&](int64_t num) -> int64_t {
+  return fastpow(num, md - 2);
+};
+```  
+Computes the modular inverse of a number $\( \text{num} \mod \text{md} \)$ using Fermat's Little Theorem. This method is efficient and operates in $\( O(\log \text{md}) \)$.  
+
 ### 3. Precomputations
 #### Factorial Table Construction
 ```cpp
@@ -42,6 +50,7 @@ void buildFactorial() {
 ```
 Precomputes factorial values up to a maximum limit (`MXN`) modulo `md`. This precomputation allows for constant-time access to factorial values, which are used extensively in combinatorics calculations.
 
+
 ### 4. Combinatorics
 
 #### Binomial Coefficients (`nCk`)