Move the "Translations" section into its own page.

[Mariatta]

I think documentation translation effort is big and important enough to have its own page, and its own heading in the sidebar menu.

Currently it is a sub-section under "Documenting" page, and we have to scroll all the way down to find it.

https://devguide.python.org/documenting/#translating

cc @JulienPalard

[encukou]

Please keep the #translating link working though (i.e. a stub section with a link to the new place).

[ezio-melotti]

Done in #901.