Move some manifests members into Web App Manifest - App Info spec

[filips123]

What was wrong/why is this fix needed? (quick summary only)

The categories, description, iarc_rating_id and screenshots members of Web App Manifest have been moved into new Web App Manifest - Application Information specification.

MDN URL of the main page changed

• https://developer.mozilla.org/en-US/docs/Web/Manifest
• https://developer.mozilla.org/en-US/docs/Web/Manifest/categories
• https://developer.mozilla.org/en-US/docs/Web/Manifest/description
• https://developer.mozilla.org/en-US/docs/Web/Manifest/iarc_rating_id
• https://developer.mozilla.org/en-US/docs/Web/Manifest/screenshots

Issue number (if there is an associated issue)

None

Anything else that could help us review it

This depends on mdn/yari#3835 to add "App Manifest - Application Information" specification to macros.

[Elchi3]

Hi, thanks for your PR! I've created #5164 to fix this instead. We are re-doing how the specification section works right now (see openwebdocs/project#24), and so it would be good if the Manifest pages make use of that.