Add signatures object to jsondoc for routine types

[PMunch]

jsondoc is meant to be read by computers, but yet the signatures of
procedures where simply a string of the whole thing. This adds a
signature object that unpacks this information into an object so it's
easier to analyse the documented signatures.

I will add a changelog entry and fix the documentation if this gets approved for merging.

[Araq]

Well I have no overview over what kind of tooling relies on "jsondoc".

[timotheecour]

maybe not much so far because jsondoc hasn't been given much attention, but it would be useful for tooling to read nim generated docs (eg IDE's, cmd line tools) ; #9462 and #8495 are relevant

[PMunch]

I haven't seen anything that uses this. What I wanted to do was to automatically generate tests based on the documentation to check a claim I make. This is also completely backwards compatible, I haven't removed any fields from the JSON, just added new ones.