Document yaml as supported language for syntax highlighting

[mportiz08]

Bug/issue #, if applicable:

Summary

Document that yaml code listings can now be syntax highlighted with DocC-Render.

Adds an entry to the table in the "Add Code Listings" section of the "Formatting your documentation content" article that documents which languages are expected to have syntax highlighting support in the renderer.

Dependencies

swiftlang/swift-docc-render#960 (thanks @heckj!)

Testing

Steps:

1. Run bin/preview-docs DocC and verify that the new table entry can be found at http://localhost:8080/documentation/docc/formatting-your-documentation-content#Add-Code-Listings

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[heckj]

@swift-ci please test

[heckj]

Thank you @mportiz08 ! Definitely great to have that update in here!