unicode: post to ICU userguide.

[srl295]

@lucacasonato and FYI @catamorphism - this task is to move the C++ / J docs into PRs into ICU4C/ICU4J/s userguide

• https://github.com/unicode-org/messageformat.dev/blob/main/docs/integration/cpp.md
• https://github.com/unicode-org/messageformat.dev/blob/main/docs/integration/java.md

into

https://github.com/unicode-org/icu/tree/main/docs/userguide/format_parse/messages

Noting recent work:

• ICU-22921 Add howto guide to try MF 2.0 final candidate draft impls icu#3334

[lucacasonato]

I feel it may be useful to have a quick start guide (with links to the full docs in the ICU user guide) in these canonical MF2 docs. For folks using ICU already a page in the ICU user guide will be useful, but for folks who learn about MF2 first may be looking for docs on how to use MF2 with their language of choice in the MF2 docs.

I am thinking the docs here should cover the quick start for all major implementations:

• How to set up (could link out)
• Code sample on loading a message
• Code sample on formatting a message by passing some custom data
• Code sample for adding a custom function when formatting

That way it is easy for users to "compare" how MF2 can be used across implementations easially.

[srl295]

@lucacasonato makes some sense. I think when this page was added, ICU didn't have any MFv2 (or didn't have much content.) Perhaps it could be a discussion w/ ICU-TC?

anyway I don't think this is a blocker for opening the repo.

[littledan]

Yeah, it'd be good to improve the docs on the ICU side, but maybe we can leave things "denormalized" with the content posted in both places.