Promote LB4 in loopback.io site

[dhmlau]

With LB4 being the current release and LB3 as LTS release, it would be great if LB4 shows up as a more prominent location in loopback.io web site.

Discussed with @raymondfeng @bajtos, we're thinking along the line of Node.js web site. See screen shot below:

Acceptance Criteria

• in the main banner of loopback.io, shows 2 "buttons" - one for LB3 and one for LB4.
  • LB4 button points to v4.loopback.io
  • possibly some text around when to use LB3 and when to use LB4, since there's some feature parity gap in LB4

cc @raymondfeng @bajtos

@joesepi, this is originated from your comment. Appreciate your input. Thanks!

[joesepi]

I think this makes perfect sense. Perhaps have a short bit of text above the buttons highlighting v4 and that v3 is "LTS" or however you'd phrase that. I could potentially help with this work after the holidays. Thanks.