Improve layout for right-to-left languages

• Set the correct direction on the <main> element (the same element that also has the language) if we can determine it, otherwise let the browser guess.
• Set the body’s text-align to start (correct for ltr and rtl), rather than Bootstrap’s default left. (Supported in all modern non-Microsoft browsers.)
• Override the browser’s default margin-inline-start for the examples – Bootstrap only overrides margin-left.