feat: add Vega-Lite docset

[hydrosquall]

Motivation

• Improve developer experience for users of the Vega-Lite data visualization library by enabling offline browsing of examples + spec documentation
• Screenshot:

Features

• See generator script for the most current listing, built using guidelines from checklist and generation guide
  • No version mentioned in Info.plist
  • All pages work offline. Some pages will look better when online (I originally tried loading the data folder of CSV and JSON too, but it appears Dash doesn't run a local fileserver, so the docs scripts unable to fetch them with XMLHttpRequest
  • Provided table of contents for all the pages under Categories using URL encoded item names
  • Removed the sidebar on the specification pages, as requested in the checklist
  • Provided 2 icons by resizing the official assets, see feat: add square Vega-Lite color logos @ 128 , 32, 16 pixels vega/logos#1
  • JS is enabled, see any of the Samples to test
  • Index page is set (Visit Main Page)
  • All links in the docset index are free of newlines and extra spacing.

[Kapeli]

Thank you! 👍

One small issue:

Found directory in main index: Entry with row id "186" and path "docs" at "vega-lite.docset"

The name of the entry is "Overview". It causes Dash to show a 404 page.

[hydrosquall]

#3383 (comment)

@Kapeli thanks! Filed an issue to look into this.

(Update: fixed in #3385 )