Add some meta-info to landing TOC page

[zlovatt]

Right now, navigating to the homepage just gives you a TOC with no info or context.

It'd be good to have a mission statement / intro up top that details:

• What the site is
• What the site isn't
• How to navigate it
• Where to go for help / corrections (this repo!)
• Info about it being a living doc / constant WIP, contributions open, etc.