docs/translating: Follow-on cleanups from removing Django templates.
docs/translating: Follow-on cleanups from removing Django templates.

This follows up the recent commit
3d1d09b3d docs: Remove discussion of old Django templating engine.
with a small grammar fix and removing another vestige of making the
distinction between Jinja2 and Django templates, and also rearranges
the logic slightly to reflect that backend and frontend templates
have separate sections.

Probably a bigger restructuring is in order to help the reader
navigate through all the good content in this doc, but that's
a bigger job for another day.

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