doc: add function-index with suggested renames toward nvme_verb_noun convention

[Copilot]

libnvme's public API uses an inconsistent naming scheme — some functions are nvme_get_host, others are nvme_host_get. This adds a reference table mapping every public function to its suggested rename following the uniform nvme_<verb>_<noun> pattern.

Changes

• New file libnvme/doc/function-index.md — two-column table (Function | Suggested Name) covering all 233 public functions across 9 headers (tree.h, mi.h, linux.h, lib.h, ioctl.h, filters.h, fabrics.h, nbft.h, util.h)
• Functions already following the convention show — (102 entries); the rest carry an explicit suggestion (131 entries)

Renaming rules applied

Current pattern	Suggested pattern
nvme_host_get	nvme_get_host
nvme_ctrl_get_state	nvme_get_ctrl_state
nvme_ctrl_for_each_ns	nvme_for_each_ctrl_ns
nvme_ns_head_get_sysfs_dir	nvme_get_ns_head_sysfs_dir
nvme_transport_handle_get_fd	nvme_get_transport_handle_fd
nvme_mi_aem_aesi_get_aese	nvme_mi_get_aem_aesi_aese
nvme_mi_mi_config_get	nvme_mi_mi_get_config
nvme_hostnqn_generate	nvme_generate_hostnqn

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