/Guide needs an intro paragraph

[fantasai]

If you land on /Guide without context -- which many people will once the new website is launched -- you need some context for what it is and how it relates to e.g. the Process and the CEPC. So we need an intro paragraph to provide such context.

[koalie]

The Guidebook jumps straight into listing types of activities and pointers but it has an "About" section at the bottom of the homepage.
I'm not convinced it's the best approach, and we intend to improve incrementally overtime, cf. #288