Perfetto <-> Ftrace interoperability

This doc is WIP, stay tuned.

This doc should:

Describe the ftrace trace_pipe_raw -> protobuf translation.
Describe how we deal with kernel ABI (in)stability and ftrace fields changing over kernel versions (we process event/**/format files on-device).
Describe how to generate ftrace protos (tools/pull_ftrace_format_files.py, tools/udate_protos.py)
Describe how session multiplexing works.