docs: Add store file format documentation (mirrored from CLI repo)

[Siddhant-K-code]

Description

Adds store file format documentation to the modeling section as a mirror of the primary documentation in the CLI repository, as requested in openfga/cli#490

Documentation Structure

• Primary source: openfga/cli/docs/STORE_FILE.md
• Mirror location: /docs/modeling/store-file-format
• Navigation: Positioned after "Testing Models" for logical flow

Mirror Notice

The documentation includes a clear notice that it's mirrored from the CLI repository and directs users to the CLI docs for the most up-to-date version.

Integration

• Integrates with existing modeling documentation structure
• Referenced from the testing models page
• Available for VS Code and IntelliJ plugins to reference

References

fixes openfga/cli#490

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected