Initial documentation for the Monero libraries (#122)

* Document all features

* Largely document the Monero libraries

Relevant to https://github.com/serai-dex/serai/issues/103 and likely 
sufficient to get this removed from 
https://github.com/serai-dex/serai/issues/102.

coins/monero/src/ringct/mod.rs

@@ -14,6 +14,7 @@ use crate::{
   ringct::{clsag::Clsag, bulletproofs::Bulletproofs},
 };
 
+/// Generate a key image for a given key. Defined as `x * hash_to_point(xG)`.
 pub fn generate_key_image(mut secret: Scalar) -> EdwardsPoint {
   let res = secret * hash_to_point(&secret * &ED25519_BASEPOINT_TABLE);
   secret.zeroize();
@@ -74,6 +75,7 @@ pub enum RctPrunable {
 }
 
 impl RctPrunable {
+  /// RCT Type byte for a given RctPrunable struct.
   pub fn rct_type(&self) -> u8 {
     match self {
       RctPrunable::Null => 0,