replaces deprecated GitHub Actions filter (#1503)
replaces deprecated GitHub Actions filter (#1503)

The method used to restrict 'docusaurus' actions to the main branch was
deprecated and replaced with a new YAML syntax.

https:39030cf4accf13d40030b6961ac60d4d0901f089@latest/actions/reference/workflow-syntax-for-github-actions#filtering-for-specific-branches-tags-and-paths

This commit uses the new syntax.
nymag/clay-kiln,nymag/clay-kiln,clay/clay-kiln,clay/clay-kiln