Split developer information out of translators page

Since we're focusing docs/technical-reference/translating-ui.md on translators, all information not related to that role should be moved out of that page into a new or existing page in the developer's section.

The only things which should remain are the links to and information on using Weblate and the following section:

The translation is best done with Weblate. Changes are pushed to OpenRefine's git repository and are included in the next release.

Run OpenRefine from the master branch and check whether translated words fit to the layout. Not all items can be translated word by word, especially into non-Ìndo-European languages.

If you see any text which remains in English even when you have checked all items, please create a bug report in the issue tracker so that the developers can fix it. Similarly report any cases where the developer's have used string concatenation or layout to compose multi-part strings, rather than using the localization facilities.

The developer info should have added some more guidance on not using string concatenation (or layout based equivalents) as well as how to add new languages which have more than two plural forms (ie singular/plural).