The state of the JS documentation was quite sad, not updated in a long
time.  With this commit, we rework a lot of the JS doc.  Most notably,
we add a section 'Javascript Cheatsheet' (help on some common
customization tasks) and rewrite the 'Javascript Reference' totally.

Note that the JS reference page also contains a reference on all field
widgets.

This work is just the beginning, it is probably not complete, but at
least we will have a foundation.