Update "Translating" page

[remibetin]

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This update aims to simplify new volunteers onboarding, by:

• Making it clearer what they can do to help, depending on their skills and wishes.
• Using a more direct language.
• Clarifying the main steps for all types of translations

I have also slightly changed the paragraph about translation updates: we inform translators when the English version is "substantively" updated, and may remove a translation if changes are "substantial".

A useful next step would be to update the start of the subpages to make the reading smoother.

I have kept the "TR & Authorized W3C Translations" as is for now, but I intend to move some of this information to the specific WCAG translation guidance later.

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