feat(ping): add mlstdb ping subcommand

[himal2007]

Summary

Adds a new mlstdb ping subcommand for probing a single BIGSdb API endpoint. It displays the HTTP status code and response body, making it easy to verify that credentials are working or to inspect raw API data without running a full update.

Changes

• src/mlstdb/core/auth.py: Added ping_url() with auth-priority logic
  (API key → OAuth → unauthenticated fallback).
• src/mlstdb/cli/ping.py: New ping Click command with --db, --no-auth,
  and --verbose options.
• src/mlstdb/cli/__init__.py: Registered the ping command.
• tests/test_cli/test_ping.py: 9 unit tests covering all auth paths and
  error responses.
• docs/usage/ping.md: New documentation page with examples and expected output.
• README.md: Brief section pointing to the ping documentation.
• mkdocs.yml / docs/usage/overview.md: Navigation and overview updated.
• Closes feat(connect): add --url option to test authenticated API responses #37

Testing

• All 24 existing tests pass. 9 new tests added for ping.