Add new fine-tuning notebooks to README

[arnavgarg1]

Adds 3 new notebooks:

1. For Llama-2-7b
2. For Llama-2-13b
3. For Mistral-7b using the blogpost Alex and I recently co-authored: https://predibase.com/blog/fine-tuning-mistral-7b-on-a-single-gpu-with-ludwig

I wanted to call them out explicitly here so that they're easy to get to from the early part of the README, and for SEO purposes

[github-actions]

Unit Test Results

  6 files  ±       0    6 suites  ±0   20m 12s ⏱️ - 19m 35s
12 tests  - 2 780    9 ✔️  - 2 770    3 💤  - 9  0 ❌  - 1 
60 runs   - 2 768  42 ✔️  - 2 761  18 💤  - 6  0 ❌  - 1 

Results for commit 58336a7. ± Comparison against base commit df6f5ef.