Documentation updates (live and pending)

Hi everybody,

I’ve pushed a bunch of clean-ups on https://docs.simplicity-lang.org/, including streamlining our Welcome pages and some other material, and removing a few obsolete or redundant pages. We also have Simplex documentation there for the first time.

The landing page and Quickstart tutorial have a pending major revision (with Quickstarts available for Rust, bash, and Python (experimental)). I have staged that revision at Simplicity is a typed, combinator-based smart contract language for Bitcoin-like blockchains, with formally specified semantics and predictable execution costs. - Simplicity (draft) (a personal github.io page) to ask for more feedback before rolling it out. That also includes a “Try It” page that lets you run some SimplicityHL code right in your browser (invented by @stringhandler), although I’m still struggling with the CSS formatting for that page, so depending on when you look at it, it may look out-of-place within the docs site.

I would love to hear any feedback on these changes or any other suggestions or requests for our developer documentation!