Mention Pluggable Transports are not a part of Arti

Problem to solve

This may seem a bit trivial, but recently I found myself seriously scratching my head when I tried integrating any pluggable transport in external code, since nowhere was it really stated that pluggable transports are distributed separately from arti or even C Tor (which makes sense in hindsight, really, but wasn't obvious for a beginner)

Having this info explicitly stated in the docs will help solve this problem.

Proposal

Just mention somewhere (ideally next to bridges example for instance) that bridges are NOT a part of Arti and you have to install them yourself.

Who can address the issue

I'll just find a nice spot for this line, ideally where it is easily visible and noticed.