'prepare' and 'install' stanza are missing documentation

[Marco Trevisan (Treviño)]

In snapcraft docs (not just snapcraft.io, also in docs sbufolder of master git repo) there's no documentation about the 'install:' or 'prepare:'options

[John Lenton (chipaca)]

Isn't that https://snapcraft.io/docs/build-snaps/scriptlets ?

[Marco Trevisan (Treviño) (3v1n0)]

Oh, weird that grepping the sources I wasn't able to find it...

Well, it's not referenced in both the 'syntax' and 'parts' guides... Nor in the advanced-usage one. Is that wanted?

[Kyle Fazzari (kyrofa)]

We're in the process of migrating all docs to snapcraft.io, so you probably should rely on that more than what's in the docs/ directory.

That said, this information is contained in `snapcraft help plugins` as well (though not the most logical place for it. It'll probably be switched around in our move to click).