docs: updated translations guide and l10n config

[vishnunandan555]

Description

This PR updates the outdated TRANSLATIONS.md guide and cleans up the redundant configuration in l10n.yaml to match the current Flutter localization setup.
I discovered these issues while working on Malayalam localization in #719.

Fixes #738

Summary of Changes

1. Removed Outdated Steps: Removed instructions to manually add locales in main.dart or l10n.yaml because flutter gen-l10n automatically scans all app_*.arb files in the directory to compile locale support dynamically.
2. Cleaned up config: Removed the unused/ignored supported-locales block from l10n.yaml to avoid confusion.
3. Updated Supported Languages & Reference Tables: Updated the documentation to show all 10 currently supported regional languages
4. Improved paths: Clarified that untranslated.txt is auto-generated in the project root directory rather than lib/l10n/ to save time for new contributors.
5. Updated Language Codes Reference: Added reference tables for all currently supported languages and other potential future additions.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking CHANGE which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (does not change functionality, e.g. code style improvements, linting)
• Documentation update

How Has This Been Tested?

• Verified by running the localizations code generator and ensuring the app compiles and all existing tests pass successfully:
• Executed flutter gen-l10n and confirmed the localizations are correctly compiled for all 10 regional languages.
• Executed flutter test and confirmed all unit/widget tests pass successfully.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• closes DOC: Update TRANSLATIONS.md to reflect current auto-generation workflow #738
• Tag the PR with the appropriate labels

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