TSVB: Update documentation to include experimental flag

[alexfrancoeur]

We need to update our docs for the Time Series Visual Builder to include the standard experimental message and flag. @simianhacker mind updating your docs with this?

Docs to update:

• https://www.elastic.co/guide/en/kibana/current/time-series-visual-builder.html
• https://www.elastic.co/guide/en/kibana/current/_featured_visualizations.html
• https://www.elastic.co/guide/en/kibana/current/_interface_overview.html

Example warnings:

@debadair do we have a standard experimental warning?

Possible example text:

Time Series Visual Builder is new and is still marked as experimental. This feature may be changed in the future in non-backwards compatible ways if required.

[timroes]

That issue outlived itself :-) TSVB is no longer experimental.