Claude review: Add documentation for Sahara protocol.

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

This adds 1241 lines of protocol specification documentation under a new `Documentation/sahara/` directory.

- **Top-level directory is unusual.** Kernel documentation for bus-specific drivers typically goes under an existing subsection. A new top-level `Documentation/sahara/` for one protocol seems out of place. Consider `Documentation/bus/mhi/sahara_protocol.rst` or similar.

- **Not wired into the doc tree.** There's no change to `Documentation/index.rst` or any parent toctree, so `make htmldocs` won't find these files. The index.rst is created but never referenced from the main doc tree.

- **The doc is very long (1241 lines) and reads more like an internal protocol spec** than kernel documentation. Consider whether this level of detail is appropriate for the kernel tree, or whether a high-level overview with a reference to the spec would be better.

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