docs: clarify rache's API model

[tisonkun]

Summary

• Add a caller-oriented API model that explains one-shot functions, incremental states, std::io::Write, Hasher, and BuildHasher.
• Add a complete algorithm capability map to both the README and crate documentation, including the deliberate CityHash and 128-bit adapter gaps.
• Make each algorithm-family module independently explain its variants, configuration, integration boundaries, limitations, and basic usage.
• Present public modules and configured constructors before crate-private implementation details in source.

Design Notes

This follows the strongest documentation pattern shared by AsyncBand and DataSketches: establish project scope, give users a task-oriented map, then make each module useful when reached directly from docs.rs. Existing APIs remain unchanged.

Associated oneshot methods are retained and documented as namespaced delegates to the module-level functions. Although they duplicate the free-function surface, removing them would trade a modest reduction in API size for a real discoverability and compatibility cost without evidence of caller harm.

No changelog entry is included because the changes do not alter supported or observable behavior.

Validation

• cargo x lint
• cargo x check
• cargo x test