Add a cheatsheet

[florentc]

Rendered version of the proposed wip cheatsheet (you can use the table of contents generated by Github).

Goals:

• Answer practical needs about how to do specific actions
• Self-contained answers
• Not to be read as a document, CTRL+F would be the way to go
• Provide very quick reminders...
• ... or provide pointers about where to look in actual documentation or code
• Not intended to replace documentation

Features:

• no text or as little as possible (not an FAQ)
• short answers
• information may not be exhaustive
• possible redundancies
• code is not runnable as is (undefined variables, holes (...)).

[Niols]

I think this is interesting to have indeed.

[florentc]

I have added some missing items after our latest audit. IMO, the cheatsheet is ready to be merged for V3.

[carlhammann]

I think the cheatsheet is really nice. It was already very useful to have around at the last audit, and now, in its more complete form, it'll be great!