Add a make command for yarn i18n, such as make i18n

[stevepiercy]

Should we add a make command for yarn i18n, such as make i18n? We've been trying to unify all commands in the Makefiles.

Here's the output from the command.

$ yarn i18n    
yarn run v1.22.19
$ pnpm --filter @plone/volto run i18n

> @plone/volto@18.0.0-alpha.42 i18n /Users/stevepiercy/projects/Plone/documentation/submodules/volto/packages/volto
> rm -rf build/messages && NODE_ENV=production i18n

Extracting messages from source files...
Synchronizing messages to pot file...
Synchronizing messages to po files...
Generating the language JSON files...
Merging @plone/volto-slate locales for de
Merging @plone/volto-slate locales for en
done!
✨  Done in 6.71s.

Here's the recently updated documentation:

https://6.docs.plone.org/i18n-l10n/contributing-translations.html#translate-volto

Originally posted by @stevepiercy in plone/documentation#1691 (comment)