Add documentation for Symmetric Translations
**Problem/Motivation**
Canvas supports symmetric translations, but there is no end-user documentation for setting them up.
**Proposed resolution**
Add the guide to the Canvas docs (e.g. `docs/translations.md`), covering:
- Enabling the required modules and configuring field translatability for symmetric translations
- Translating pages, content templates, and global regions via TMGMT
- The experimental Canvas Translate module as an alternative UI
Link it from the docs index.
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