Add yaml block validator to CI

[rgeniesse]

This is an issue with:

• [ ] Bug (site functionality or styling)
• [ ] Errata (fix needed for doc content)
• [ ] New content (guide wanted)
• [ ] Update (Add missing or refresh existing content)
• [X] Enhancement (add new site functionality)

Current Behavior

Currently there is a json block validator that runs with CI. To go along with the eventual move to yaml examples with Sensu Go, a yaml validator should be written that does the same as the current json one. Language does not matter.

Possible Solution

Write it!

[hillaryfraley]

https://github.com/oxsecurity/megalinter