docs: Add initial documentation setup with Sphinx; Add task automation for docs building.

[junhaoliao]

Description

This PR sets up the docs infra of the zstd-ffi-js project, using https://github.com/y-scope/spider as a reference.

Checklist

• The PR satisfies the contribution guidelines.
• This is a breaking change and that has been indicated in the PR title, OR this isn't a
  breaking change.
• Necessary docs have been updated, OR no docs need to be updated.

Validation performed

1. Ran npm run docs and observed the docs site dev server was launched.
2. Visited http://127.0.0.1:8080/ in a browser and inspected every page. Found no broken link.

[Henry8192]

LGTM.