[Docs] Add task-oriented sections to Vega docs

[wylieconlon]

I've recently written an entire dashboard of Vega visualizations that imitate the features of existing Kibana visualizations, and found some specific improvements to the Vega docs. In particular, we are missing a "fully-featured" example that supports the following features, and is able to show the same example in both Vega and Vega-lite.

• Aggregated query that combines common aggregations like date histogram, filters, and terms
• Using the "property" to extract buckets from the response
• Using the "flatten" transform to convert the data into something Vega can use
• Using a toDate function to parse dates that are in string form
• Using the "stacked bar" or "stacked area" encoding
• Adding a nice-to-read tooltip encoding such as "tooltip": "{ 'Date': format(datum.key, '%m %d %Y') }"
• Adding hover effects using events in Vega and selections in Vega-lite, especially for a line chart
• (Vega only) Updating the Kibana dashboard filters and time range more details
• Tips for debugging the visualization, using VEGA_DEBUG
• A second example for a basic non-aggregated query would be useful, but I think it's optional

I think the best Vega examples we can build are basic bar or line charts that are as close as possible to existing Kibana charts. Having a complete and detailed example of a basic stacked bar chart, with the same code available in Vega and Vega-lite, is the missing documentation that I've been looking for.

I haven't copied+pasted my sample dashboard into this issue yet because I need to do some cleanup, but first I want to see if this goal is one that makes sense

cc @nyurik @KOTungseth

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)