Extend documentation for memory providers

[vinser52]

Description

• Today Memory provider API section contains only APIs from the memory_provider.h header.
• But also we should describe the umf_memory_provider_ops_t structure. It contains mandatory and optional interfaces (callbacks to implementation).
• Each memory provider implementation should describe which interfaces it implements. For example, L0 provider implements IPC but does not implement umf_memory_provider_ext_ops_t.