chore: add metadata for canonical version of documentation + warning on 'outdated version'

[mcdurdin]

Coming out of Chrischona discussion M4:

From Lorna and Marc: Improve Keyman documentation metadata so that searches return the latest version of the docs. Also mark older versions of documentation with a warning that it is not the latest and provide a link to the latest.

This was again brought up at the Chiang Mai 2024 conference. It may need noindex meta tag for the duplicated older versions not to be recorded in the web searches.

Part two from Chiang Mai 2024 Is to add noindex meta data to less important pages and search pages that don't need to be indexed by search engines.

[mcdurdin]

Some of this is already in place -- we have a rel=canonical tag on older content already.

[mcdurdin]

Unassigning myself for now on this.