Improve SEO with `html_meta` directive.

[stevepiercy]

All files now have placeholders for content to be added to the html_meta tags.

See plone/training#519

• Volto
• plone.api
• plone.restapi
• backend
• Classic UI

[stevepiercy]

Volto has been completed.

[stevepiercy]

plone.restapi has been completed in plone/plone.restapi#1382

[ksuess]

done: plone.api plone/plone.api#486