User’s guide: Information for JS developers

[nomeata]

The new user’s guide, added in #158, does not fully address the JS-specific workflow and type mappings.

Story

As a JS frontend developer, I want JS-specific Candid documentation, so that I can build JS frontends that use Candid well.

Demo

• Go to https://github.com/dfinity/candid/blob/master/docs/modules/candid-guide/index.adoc
• Section “How do I use Candid as JavaScript front-end developer?” has useful information
• In the reference section, all types describe their mapping to JS values

[lsgunnlsgunn]

The 2pts is for updating
https://github.com/dfinity/candid/blob/master/docs/modules/candid-guide/pages/candid-types.adoc
corresponding JS type sections.