Simplify gRPC API navigation

[danielporterda]

Summary

Addresses #309 by updating the generated gRPC API navigation shape.

Screenshots

Pending hosted Mintlify preview screenshots. This PR is draft; screenshots should show the Ledger API > gRPC API sidebar after the preview deployment is available.

Changes

• Removes the Packages wrapper under Ledger API > gRPC API.
• Removes each package's Services submenu.
• Keeps service groups directly under v2, v2.admin, v2.interactive, and v2.testing.
• Moves each package overview page to the bottom of its package group as Details and history.
• Adds focused regression coverage for the generated gRPC nav shape.
• Regenerates checked-in gRPC reference nav and package detail pages.

Rebase note

This PR is now based directly on main at 9a28ce6c. The branch contains one commit after the rebase: 04778306 Simplify gRPC API navigation.

Validation

• direnv exec /Users/danielporter/control/.worktrees/docs-issue-309-grpc-nav python3 -m pytest tests/test_grpc_ledger_api_nav.py
• direnv exec /Users/danielporter/control/.worktrees/docs-issue-309-grpc-nav npm run generate:all-reference-docs
• Generated-nav jq check confirmed no Packages or Services labels remain under Ledger API > gRPC API, and each package detail page is last.
• git diff --check

Note: full direnv exec /Users/danielporter/control/.worktrees/docs-issue-309-grpc-nav python3 -m pytest tests currently reports 34 passed, 3 failed; the failures are the existing relative-link expectation mismatches in tests/test_characterization_outputs.py, tests/test_jvm_docs.py, and tests/test_protobuf.py.