Add instructions for new adding locales to docs

[fozziethebeat]

Now that we have full i18n support for the website, it would be super helpful if we had clear step by step instructions for what files need to be added or changed in order to create a new locale. This was asked in the Discord and the person managed to figure it out, but a guide would make this much smoother.

[rjmacarthy]

I'm interested in writing this documentation.