Some layout change requests for docs.substrate.io

[lisa-parity]

I'd like to request:

• [ ] Replace Previous and Next buttons and text with icons.
• [ ] Add the "Ask a question" text link (like Submit an issue) for a link to the Stack Exchange (https://substrate.stackexchange.com/)
• [ ] Move Submit an issue to a parallel location with Ask a question, if possible
• [ ] Possibly restore the combined Edit this page and last edited date, or just have Edit this page to save on real estate in the breadcrumbs area.

but nicer than this: