refactor: Improve host handling and add comprehensive documentation

[qjerome]

Summary
Comprehensive refactoring of host-related functionality in Python APIs, adding documentation, fixing architectural issues, and updating tests. This fixes issue #19.

Motivation
The Python API is missing a way to use the parsed Host from a Url without reparsing it again (see #19).

Changes

Rust Library:

• Make Url.host field pub and document it

Python Bindings:

• Added Display trait implementation for Host enum
• Added Clone derive for Host enum to fix PyO3 compatibility
• Added methods for Host struct Host::domain(), Host::subdomain, Host::suffix + documentation
• Updated FaupCompat methods to use proper Host method calls instead of direct field access:
  • get_domain(): Now uses host.domain()
  • get_subdomain(): Now uses host.subdomain()
  • get_host(): Now returns Option<String> with proper string conversion
  • get_tld(): Now uses host.suffix_ref()

Type System:

• Updated Python type stubs (pyfaup.pyi) to reflect new method signatures
• Changed Url.host type from Optional[str] to Optional[Host]
• Removed direct field accessors (subdomain, domain, suffix) from Url class in favor of method calls on Url.host object

Tests:

• Updated existing tests to use new Host method calls
• Added comprehensive tests for Host.domain(), Host.subdomain(), and Host.suffix() methods
• Tests cover hostname, IPv4, and IPv6 scenarios with proper None handling

Breaking Changes

• Python API: Url.host is now Optional[Host] instead of Optional[str]
• Python API: Direct field access to subdomain, domain, suffix on Url no longer works - use method calls on Host instead
• Python API: FaupCompat.get_host() now returns Option<String> instead of Option<&str> (but this should be transparent for python usage)

Testing

• Rust library: All existing tests pass (40 tests)
• Python bindings: Updated tests validate new functionality
• Type checking: Python type stubs updated and validated