[Vega] Tutorials should be updated to include new inspector

[wylieconlon]

Now that we have an easy-to-use Vega inspector merged, the Vega-Lite and Vega tutorials need to be updated in a few places:

• The link to "debugging skills" should instead link to the Inspector
• The tutorials shouldn't reference the browser dev tools at all, and instead provide instructions for debugging using the inspector
• The reference to the "flatten" transform should be updated post-7.9
• The Vega tutorial doesn't use the debugging tools at all, but it could use them to inspect the Signal values during the tutorial

cc @alexwizp

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

The inspector just got merged for 7.10, so it would be good to update the vega docs for 7.10 forward

[alexwizp]

@dziyanadzeraviankina could you please update docs?