Spell / step documentation incomplete

[johnsca]

It seems that the documentation for creating spells and steps are out of date and missing some newer features, most notably the ability to add and remove fragments from the bundle in steps.

We also need to consolidate on one set of docs, as there are currently some docs on the conjure-up repo's wiki, the spells repo's wiki, and the conjure-up User Manual (which is also still at rev 2.4.0).