Improve how documentation content is presented

[milesfrain]

We have good content in this repo, but I think it could be easier to absorb and navigate if its presentation was improved.

I like what ReasonML does for their docs, and propose we adopt some of their strategies:

• A concise overview page of short examples, with a "details" link to each topic for a deeper explanation.
• Presented on a dedicated webpage with a navigation sidebar. This is much nicer to use than clicking through a tree of files on github.
• A smart search feature with prioritized results.

[milesfrain]

Beginners can also get pretty lost with unfamiliar syntax, so I'd like to provide a way to find the relevant explanations.

For example:

Symbol or Keyword	Used For
do	"do notation"(link)
<-	"do notation"(link), "pattern guards"(link)
->	"basic functions"(link), "lambda functions"(link), "case expressions"(link)
.	"record access"(link), "generic functions / forall"(link)
::	"type signatures"(link)
\	"lambda functions"(link)
_	"ignored type"(link), "anonymous argument"(link)
#	applyFlipped / "kinds"(link)

Originally proposed in https://discourse.purescript.org/t/trying-to-modify-a-dice-demo-then-got-stuck/1689/3