Exploring presentation of links to informative materials, toggletip edition

[hidde]

This builds on #685 and gives each guideline and provision a toggletip for 'info', that can contain related content on requests.

Caution

Best previewed in recent Chrome or Edge; demo uses modern CSS that will be available across all browsers this year; we can polyfill / add fallbacks to make it work for older browsers too

Benefits:

• one tab stop for however much related content we want to link out to
• it floats so does not cause reflow
• (I'm hoping) having it available on request is good rather than always; not in the way for people who don't need it, easy to find for people who do

This specific implementation:

• uses HTML/CSS only
• has accessibility semantics, keyboard behaviour (ESC for close)
• needs explicit open/close, avoiding issues of having to make hover behaviour work on touch devices (this 'just' works with touch)
• can be polished eg to automatically make it flip in right direction depending on available space, and on smaller screens

Open questions:

• What actual content to add… is it just a list of links? is a heading or introductory text ('this info is available: ') helpful?