Document Nargo.toml

[Savio-Sou]

As we could be introducing more fields in Nargo.toml as time goes on (e.g. noir-lang/noir#2056 added name), it might be a good idea to describe fields in Nargo.toml in a page / section in the docs.

[critesjosh]

@Savio-Sou is this to document with the 0.9.0 or 0.10.0 release?

[Savio-Sou]

Theoretically it applies to all versions (as we have always had Nargo.toml).

Practically let's document it starting v0.10.0, not much value in retro-doc'ing old versions of the file.