Headline documentation should endorse h1-h6

Headlines should use <h1> ... <h6> tags to provide semantic code for headings in the content. Heading markup will allow assistive technologies like screenreaders to present the heading status of text to a user.

The default tag of a Stripes Headline is <div>.

The Headline documentation https://github.com/folio-org/stripes-components/tree/master/lib/Headline should endorse not to use the default <div> but <h1> ... <h6>.

Properly using <h1> ... <h6> is a WCAG 2.1 Level A requirement:

• https://www.w3.org/WAI/WCAG21/Techniques/html/H42 "Using h1-h6 to identify headings"
• https://www.w3.org/WAI/WCAG21/Techniques/general/G141 "Organizing a page using headings"
• https://www.w3.org/WAI/WCAG21/Understanding/info-and-relationships "Understanding Success Criterion 1.3.1: Info and Relationships"

Only 153 out of 263 <Headline> usages change the <div> tag, see attached headline.txt file:

cd platform-complete/node_modules/@folio
grep -rPazo "<Headline[^>]*>" | tr \\0 \\n | grep Headline | wc -l
263
grep -rPazo "<Headline[^>]*>" | tr \\0 \\n | grep "tag" | wc -l
153