Add a 'Basic Overview' docs section

[nathanl]

I've been meaning to wrap my head around Gettext for a while. When I first looked at it, I was confused about how the English string could be the message id without that being very brittle. Now that I understand better, I wrote this introduction which (among other things) explains that.

[whatyouhide]

Love this, thanks @nathanl! 💟

[nathanl]

@whatyouhide I also just started a repo for community translations of common strings (forgive me if this already exists somewhere): https://github.com/nathanl/elixir_common_i18n/tree/main/gettext