* add proof for split exact sequences

* port exercise R.2.9

* Noetherian rings: add ex. 3.2 & 3.5

* Hilbert basis theorem, Noetherian module properties

* add Noetherian exercises 3.3 & 3.4

* port initial notes on commutative algebra: ideals & modules, Nakayama's lemma, etc

* port notes on Noetherian rings&modules

* add ideals related definitions

* improve Cayley-Hamilton proof (specially determinant trick explanation)

* polishing

* add typos detection

* add some exercises, and proof of Z and K[X} being PID