Update NPM installation instructions for Sentry CLI #15788
Description
The NPM installation instructions contain some information which may be out-of-date following getsentry/sentry-cli#1833, which publishes the Sentry CLI binaries to NPM. By default, we pull these binaries, fully bypassing the install script. I specifically suspect that the "Downloading From a Custom Source" and "Available Installation Options" may not work as advertised.
Let's verify what is and is not accurate, and adjust accordingly.