Out of date jquery library

[martinohanlon]

Cinder is used for the docs site of guizero.

@lawsie received a security notification about the use of an older version of jquery (the version used in cinder).

We tried updating the jquery library used by cinder from 1.12.4 to 3.6.4 and perhaps not unsurprisingly given hindsight saw issues, the key one being the navigation of drop down menus stopped working.