Revise resources for markdown & GitHub (fixes #807)

[JLKwong]

Fixes #807

Description

Moved resources around to make readings more concise and easier to follow.

1. Deleted Preparation, AND added Readings
2. Replaced GitHub's hyperlink with a link to a more concise summary/reading for GitHub, much like the ones for Markdown and MDwiki. (https://education.github.com/git-cheat-sheet-education.pdf)
3. Moved 2 of the markdown resources inside the note at the bottom of the page, adding text that these are additional resources for markdown. I left the other 2 markdown resources because they are interactive (active learning promotes long-term learning, especially for syntax).
4. Moved the git cheat sheet into a hyperlink for the word 'git' in the description for GitHub.

Raw.Githack preview link

https://raw.githack.com/JLKwong/JLKwong.github.io/807-Revise-Resources-Markdown-GitHub/#!pages/vi/githubandmarkdown.md