Visibility and Promotion of Boost Software License

As the Boost Software License is a key asset of the library collection, it should be featured more visibly on the website. Some ideas for this

* Top level navigation "License"

* Placement at the top of the home page

* Prominent shortcut link in the User's Guide card on the Learn page

* Top level Table of Contents item in the User's Guide

* Description at the top of the Libraries page and all individual Library pages:
"All library source code is proffered under the Boost Software License"

* A whole site-doc focused solely on the Boost Software License