docs: Mention that downgrading dependencies might be needed at times

[matthewfeickert]

Description

Given the problems brought up with click v8.X in Issue #1506, it is probably worth noting in the docs FAQ section that if a user runs into a problem with a core dependency acting weird that as pyhf is a library and not an application we don't overly control the range of dependencies and they might need to manually downgrade dependencies, e.g.

# requirements.txt
click<8.0.0
pyhf==0.6.2

, especially if there has been a recent release of the dependencies since the last release of pyhf.