docs: add info about versioned traceconv downloads
docs: add info about versioned traceconv downloads

Running:

curl -LO https://get.perfetto.dev/traceconv

downloads the latest traceconv binaries. This is not reliable because
any sort of breaking change could harm users. Hence, this change
documents the recommended source for downloading specific versions of
the traceconv binaries.

Signed-off-by: Darshan Sen <raisinten@gmail.com>

Change-Id: I35253fd7bfd61611694aac5bd7486f05ba7db881

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto