Simplify gRPC API navigation

[danielporterda]

Summary

Addresses #309 by updating the generated gRPC API navigation shape for the current Mintlify products navigation.

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.
• Updates the gRPC nav updater and shared Ledger API regrouping helper to support the current navigation.products layout as well as the older navigation.dropdowns layout.
• Adds regression coverage for the flattened generated gRPC nav shape and for preserving the separate Admin API gRPC section.

Screenshots

gRPC API details page	v2 service navigation

The first screenshot shows Ledger API > gRPC API without the extra Packages wrapper. The second screenshot shows the v2 package with service groups directly under it and Details and history at the bottom of the package group.

Validation

• direnv exec /Users/danielporter/control/.worktrees/cf-docs-pr-346-check python3 -m pytest tests/test_grpc_ledger_api_nav.py
• direnv exec /Users/danielporter/control/.worktrees/cf-docs-pr-346-check npm run generate:grpc-ledger-api-reference
• Generated-nav check confirmed no Packages or Services labels remain under Ledger API > gRPC API, each package detail page is last, and Admin API gRPC navigation remains present.
• git diff --check
• Local Mintlify preview at http://localhost:3072/reference/grpc-ledger-api-reference/details and http://localhost:3072/reference/grpc-ledger-api-reference/com-daml-ledger-api-v2/commandservice/submitandwait

Note: full npm run generate:all-reference-docs currently fails on current main outside this PR's gRPC path because other reference generators still expect docs.json navigation.dropdowns after the site moved to navigation.products.