Add documentation for rpc diagnostic command

[ShradhaGupta31]

Update reference section with details of rpc diagnostic command & its subcommands

[ShradhaGupta31]

proposal doc in review - https://github.com/device-management-toolkit/rpc-go/wiki/RPC-Diagnostic-Command-Design-Document

[sinchubhat]

Documentation for diag cmd is included as part of #769
I have informed ganesh on the minor changes needed on the PR.