docs/website: make "outdated version" banner red if ancient (#5339)

Where "ancient" means it's not among the latest five releases.

Also, in, netlify.toml: declare framework for dev. I've found that
adding this live means I can use

    netlify dev --offline

without any further switches. For some reason, I had run into
trouble with "cecil" when using it like that.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>