Docs/update cmdstager

[bwatters-r7]

This is an update for the cmdstager documentation. It adds examples, clearer descriptions, and goes into more depth in the how-to guide and deletes certutil from the list of cmdstager flavors that require SRVHOST and SRVPORT.
Yes, certutil does allow us to fetch a payload from an http server, that's not how we use certutils: https://github.com/rapid7/rex-exploitation/blob/c0b3b64d1a8f5b85a5d1bd0660f2a44590dd5f8f/lib/rex/exploitation/cmdstager/certutil.rb#L38
With the certutil cmdstager, we echo the payload into a file as a certificate then use certutil to decode it; I don't see anywhere that we use certutil to download a payload. I also added that info to the certutil description in the how-to docs because I was pretty sure we downloaded the payload using certutil, too.
Honestly, when I did a deeper dive a while back, I was pretty shocked most of the non-fetch based cmdstagers just used echo. It's there in the code, but I tried to hammer that point home in the docs.

[adfoster-r7]

@bwatters-r7 That PR #17849 is merged now; If you rebase against master, this PR should go green now 🤞

[bwatters-r7]

Fixes #9209

[adfoster-r7]

Release Notes

Updates the How to use command stagers documentation with additional examples and clearer descriptions