feat: add Greek (el), Finnish (fi), Serbian (sr) language navigation

These 3 language folders existed with complete translated content
(121 files each, identical structure to en/) but were missing from
docs.json language configuration, making them invisible to Mintlify.

- Added navigation blocks for el, fi, sr with translated tab/group names
- All referenced .mdx pages verified to exist on disk
- Now 18 languages total (matching PlatformBackend's 18-language support)

Author: acedatacloud-dev