Now that we've introduced a new usei18n composable to Codex (which is expected to become our standard way of handling translatable strings in the library going forward), we need to provide some developer-facing documentation on how this system is to be used when authoring components.

The "Developing Components" page in the Codex docs is probably where this information ought to live – ideally in a new dedicated subsection on the page. The documentation should contain some code examples, links to relevant parts of the source code, and notes on how things would integrate with MediaWiki.

A couple of paragraphs plus examples should be sufficient.