Heading render hook: support all attributes

[chalin]

• Fixes Heading render hook should support all attributes #2205

The only change to the generated UG pages is due to the removed .test-class:

$ (cd public && git diff -bw --ignore-blank-lines -I "dateModified|modified_time|data-proofer" -- ':(exclude)*.xml') | grep -v _print | grep ^diff
diff --git a/docs/adding-content/navigation/index.html b/docs/adding-content/navigation/index.html

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