Convert markdown files to markdown

There is a **ton** of good content in here! Well done, everybody! :smile: 

* Converted the git README.md from HTML to pure markdown (this should happen for all the files).
* Added links to each doc from main readme.
* A few minor content tweaks.

BTW, I used the content in the Git intro to make sure I did this pull request properly, so someone has already benefitted from your hard work. :wink: