Link to anduril.org from user guide

[Ingrid Schulman]

A link or something at every page/somewhere globally in the user guide so that you can get back to the main page.

[Ville Rantanen]

as mkdocs itself doesnt support extr. links in the menu, the only way to do it is to add a custom theme. which we have for the anduril.org site. adopting the theme will make the user guide visually similar to andurilorg, which is a nice thing.

[Ville Rantanen]

done