Add ecFlow commands technical note

Add a technical note under docs/tn/ documenting the full ecFlow
client/server command set, and link it from the Technical Notes index.

It groups the 35 commands into task commands and user commands, and
documents for every command its type, C++ class, command-line action(s)
and parameters, environment variables, usage, and the native cereal
request/reply payloads exchanged on the wire.
Each command also carries a Help tab reproducing the verbatim
ecflow_client --help output, with a combobox to switch between actions
on multi-action commands.

The goal is to collect implementation details, rather than provide user
facing information, thus it is not linked from the main documentation
index.

Author: marcosbento