Document Vega's spec schema (multiple tasks)

[zuhao]

Vega's lack of documented schema makes it hard to keep track of changes happening with the JSON spec.

It would be absolutely awesome if we can properly document (and maintain) the spec schema, so that Plotrb stays up-to-date with Vega.

Some discussions already happened here.

Be aware that Vega's wiki might be outdated. So we will have to dig into the actual code to make sure we don't miss anything.

The documentation of each component is suitable to be a single GCI task. Therefore, we would have 7 tasks here.

1. Visualization
2. Data
3. Data Transform
4. Scales
5. Axes
6. Legends
7. Marks

[translunar]

This may need to be broken down into component tasks. Each task should take ~2 hours.