include pipes in docs

[matthewlouisbrockman]

We've updated the CLI to allow piping into exec, e.g.

cat file.bin | e2b sandbox exec <id> -- python3 -c 'import sys; print(len(sys.stdin.buffer.read()))'

Updates https://e2b.dev/docs/cli/exec-command to include a snippet on piping commands

[beran-t]

lgtm 💯