add docs for new_tab functionality with regard to WCAG compliance

[johrstrom]

WCAG guidelines say you shouldn't open links in new tabs, so this documentation should reflect that/also say the same.

This here from the customization page as an example:

nav_bar:
  # Single top level link
  - title: "Title"
    icon: "fa://desktop" # optional, defaults to fas://cog
    profile: "team1"
    new_tab: true # optional, defaults to false

Note this also applies to the manifest.yml documentation page, wherever that is. Related to OSC/ondemand#5115