Diplomat generated Dart docs should point to right version

[mosuem]

The docs currently point to latest, for example https://docs.rs/fixed_decimal/latest/fixed_decimal/struct.FixedDecimal.html#method.from_str, not actually used version.

[sffc]

Some thoughts from @robertbastian @Manishearth:

• We need to parse Cargo.toml in the diplomat-gen script (or cargo-metadata)
• Needs to be done while publishing, after version numbers are bumped in the repo

[gourav-k-shaw]

@mosuem @sffc
may you please assign this issue to me
Motivation for GSOC participation brings me here. Looking to make some contributions meanwhile to explore the icu4x codebase.
Thanks

[robertbastian]

The task here is to configure the DocsUrlGenerator which is currently defaulted:

icu4x/tools/make/diplomat-gen/src/main.rs

Line 30 in 5a3ee1f

&Default::default(),

If the version of icu_calendar is 2.1.0, then the URL for icu_calendar should be https://docs.rs/icu_calendar/2.1.0/icu_calendar/. The versions need to be determined using cargo metadata.