Restrict allowed tags for heading for Subhead and improve docs

[khiga8]

What

• Restricts allowed tags for :heading slot to :div and <h1>~ <h6> elements.
• Updates documentation with best practices and additional usage information based on primer css
• Create method to link to best heading practices since it's repeated throughout codebase.

Why

Part of #491

Notes
There are 21 instances of Subhead component that explicitly set tag on heading slot and all the tags appear to be heading elements so no update needed there.

Grepped for:

Primer::SubheadComponent.new(((?!%>).)|\n)*(mb:)+(((?!%>).)|\n)*%>(((?!%>).)|\n)*component.heading(((?!%>).)|\n)*(tag:)+

[vercel]

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🔍 Inspect: https://vercel.com/primer/view-components/CfWB92yJBfTEuCYKKK8DLTgmarrb
✅ Preview: https://view-components-git-kh-updatesubhead-primer.vercel.app