Change banner for loopback.io documentation site and apidocs.loopback.io

[dhmlau]

Description

Related to: strongloop/v4.loopback.io#52
As part of the unified web site experience story, we will change the banner for the following sites to the current LB4 theme.

Acceptance Criteria

For docs site (https://loopback.io/doc), apidocs site (http://apidocs.loopback.io/),

• change the banner color to LB4 color scheme: https://github.com/strongloop/v4.loopback.io/blob/master/blueprint/general-specs/lb4-spec-color.png
• use LB4 logo
• possibly the font (IBM Plex Sans)

[dhmlau]

Closing as done.