touch up docs to link to eachother

# summary

I've toched up the documentation to correctly link to each other and use markdown syntax where appropriate.  This way, a user can start from the top of the readme, and end up at the end of the start guide without having to comb through the source tree for the relevant documentation.

# demo

You can see how the documentation will look on my forked repo https://github.com/bahamas10/ProjectTox-Core/tree/dave-touchup-docs ... scroll to the bottom to see the link to `Installation`