Add RAG Made Simple (Books, T8) and DiamantAI newsletter

[NirDiamant]

Two on-topic additions, each placed in its single strongest-fit cell per CONTRIBUTING:

1. Books > T8 RAG: RAG Made Simple by Nir Diamant - a visual, code-free guide to 22 RAG techniques. Inserted in alphabetical order between Building LLM Apps and RAG-Driven Generative AI.
2. Newsletters: DiamantAI - practical AI engineering and GenAI for builders.

Followed the entry format (icons, one informative sentence). Used a hyphen rather than the em-dash separator as a personal style preference; happy to switch to the em-dash to match the list. Disclosure: I am the author of both.

[zhimin-z]

thanks, please make it sort