build: add script for generating offline docsets

[hydrosquall]

Motivation

• Vega equivalent of build: add build script for generating offline docsets vega-lite#7642
• Enable offline browsing of Vega docs, even when browsing directly from a filesystem and not a webserver

Changes

• Added a script to package.json along with a custom copy of the jekyll build config
• Add switches to toggle SEO tags + the base URL used for fetching data (Due to browser security constraints, you can't use HTTPRequest to fetch files from the file system without a live server).

Testing

• Works together with the output from https://github.com/hydrosquall/vega-docset-generator