Documentation updates for Gerbil v0.19

[vyzo]

~Updates for all the new stuff in Gerbil v0.18.2~

It's time for v0.19 actually, pending the work on typed Gerbil.

TBD:

• finish reference/gerbil/core docs
• flesh out reference/gerbil/core/expander docs -- only the "public" api, available in the expander runtime needs to be documented.
• flesh out reference/gerbil/core/runtime docs -- only the "public" api, the externs in the core prelude, need to be documented
• document the compiler.... maybe we can punt on this.
• update the guide for typed gerbil stuff -- needs new section
• update guide etc for the compilation model; static now, no more libgerbil
• fix dangling doc references

Actually typed gerbil is undergoing major changes, so we'll have to wait a bit to finalize those parts.

[netlify[bot]]

❌ Deploy Preview for elastic-ritchie-8f47f9 failed.

Name	Link
🔨 Latest commit	b0fd8bd2b1f163727777fa395d870f471c1d5134
🔍 Latest deploy log	https://app.netlify.com/projects/elastic-ritchie-8f47f9/deploys/693ac185c4e4400007a7bbc1

[fare]

Rebased and pushed docs for atomic promises