Add i18n and l10n documentation

[stevepiercy]

Various packages have their own i18n or l10n implementation details. Yet we need to have one place to present i18n and l10n core concepts. Thus let's present core concepts at the root level, with references to implementation details to each package. Roughly an outline like this:

i18n and l10n

Common concepts

• gettext
• translation strings
• locales
• another concept

Implementation details

• Volto https://6.dev-docs.plone.org/volto/recipes/i18n.html
• plone.restapi https://6.dev-docs.plone.org/plone.restapi/docs/source/usage/i18n.html
• other packages

That will save a lot of duplication of effort, and make it easier for package maintainers.

If other core concepts pop up, we can group them in navigation. Or not. Maybe it is better to keep navigation flat.

Originally posted by @stevepiercy in https://github.com/plone/documentation/issues/1265#issuecomment-1171170715

[stevepiercy]

Ping @erral @jensens. I created a new feature branch add-i18n:

https://github.com/plone/documentation/tree/add-i18n

This branch contains the Plone 5.2 i18n documentation converted from reStructuredText to MyST and cleaned up, but waaaaaaay out of date and full of inaccuracies. Let's work on that branch to start, keeping only the current Plone 6 bits.

Next step, I will create a PR against that branch in which I will create a skeleton structure for the Plone 6 bits per my previous comment.