Add support for viewport scaling to exported HTML about octo

[pushfoo]

This is still relevant in 2023, but the nature of the problem has changed since itch's embed handling appears to have changed.

Example:

Vertical view	Horizontal view

I think there might be two steps to this:

1. Add documentation on how to set up an itch.io page with correct browser viewport sizes (#179)
2. See if there are ways of dealing itch's weird viewport behavior