Automatic Instrumentation: docs improvements and sanity check

After wasting half a day trying to figure out why even the simplest auto instrumentation examples for several different frameworks don't work (i.e. they don't emit anything to the console or the OTEL collector), I eventually found the reason: the opentelemetry-distro package was not installed. So two issues:

• The docs mention The opentelemetry-distro package installs the API, SDK, and the opentelemetry-bootstrap and opentelemetry-instrument tools and then proceed to describe the opentelemetry-bootstrap command, which seems like a "quick start" shortcut that installs a bunch of opentelementry-* packages that I don't care for. All I (thought I) needed was opentelemetry-instrument and a few handpicked packages for the libraries/frameworks I want to instrument (plus their dependencies). It is not obvious at all that opentelemetry-distro is required for auto instrumentation.
• There's no error, warning or other indication that something is missing when running opentelemetry-instrument, it just defaults to a no-op. I'd expect the auto instrumentation logic that currently lives in opentelemetry-bootstrap to be in opentelemetry-instrument, or if for some reason that's not possible or desirable, to get at least a warning if it's missing.

Start by reviewing the automatic instrumentation documentation around opentelemetry-distro, opentelemetry-bootstrap, and opentelemetry-instrument. Then inspect the behavior of opentelemetry-instrument when opentelemetry-distro is absent. Done means the required package is clearly documented and the missing-package behavior is explicitly addressed with the project’s chosen outcome.