feat: Update documentation to add IcuTrans and explain how the babel macro uses it

[cellog]

This follows up on code commits related to i18next/react-i18next#1869, and closes that issue. I've added new docs for IcuTrans, and updated the babel macro docs for icu.macro to explain that it compiles to this new component under the hood.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[adrai]

https://react.i18next.com/latest/icu-trans-component