Docs: Version Banner for /v/<version>

Creates a banner displayed on all child pages of /v/, displaying the Brooklyn version associated with the page, noting that the content may differ between versions, and offering the user three choices: View the latest version, View all versions, or Don't show this again.

'Don't show this again' implementation is by cookie. The warning must de dismissed on a version-by-version basis. i.e. if the user dismisses the warning wrt v0.4.0, it will reappear on a page about v0.5.0-M2. (Where it can be dismissed for 0.5.0-M2, and will not subsequently appear for 0.4.0 or 0.5.0-M2.)