Update documentation for precompiling the messages

[brawaru]

Is your request related to a problem? Please describe.
@braw/rollup-plugin-icu-messages was deprecated in favour of @vintl/unplugin.

Describe the solution you'd like
Documentation should be updated to use @vintl/unplugin. It should be relatively easy to update, as some of the features got merged right into the unplugin, and it's overall based on the same code.

Describe alternatives you've considered
N/A.

Additional context
N/A.