docs: add a page for translating Open WebUI

[silentoplayz]

Summary

Adds a Translating Open WebUI page for people who use Open WebUI in their own language, keep seeing English labels, and want to fill them in. The Translate the UI section on the Contributing page only explains how to add a new language, but almost all of the work is in the languages Open WebUI already ships. Every release adds strings that arrive untranslated in every language, and some languages are still mostly English.

The page covers, in the order a translator needs it:

• Find what's missing in your language: which file holds the language, that an empty value is shown in English, and a one-line command that counts the empty values.
• Translate the strings: where the English text is, including the dotted keys such as settings.personal.general.language.label, whose English is the value in en-US rather than the key. Then the rules that keep a translation working: placeholders, plural forms, names left untranslated, the file's existing terms, and changing only that language's file.
• A fluent speaker's read before submitting, since machine translation gets short labels wrong without context.
• Open the pull request: one language per pull request, the i18n: title, and the i18n checkbox in the template. A translation PR without that checkbox is closed automatically.
• Add a new language: the four steps from the Contributing page, moved here.

The Translate the UI section on the Contributing page keeps its first sentence and links to the new page, so the steps live in one place.

Related issue or discussion

None.

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

• Every statement about the translation files is checked against dev. An empty value falls back to English, and npm run i18n:parse writes the file of every language in languages.json.
• The count command gives the same number as counting empty values in the JSON, checked on four languages. Ukrainian has the _few and _many keys the page names.
• The page renders under npm run start with no console errors, and scripts/check-index-links.mjs passes.
• The page only describes the translation files and the pull request template, so it needs an update only when one of those changes. Removing it means restoring the four steps on the Contributing page.

[Classic298]

this is really not needed