Fix all broken references in docs by enabling nit-picky mode

[kratsg]

This is needed and was missed in #1257 . The -n will be useful later when we need to check for broken links (atm, this is useless since we have too many unlinked tensor calls in the docs).

Originally posted by @kratsg in #1258 (comment)

-n

Run in nit-picky mode. Currently, this generates warnings for all missing references. See the config value nitpick_ignore for a way to exclude some references as “known missing”.

Reference: https://www.sphinx-doc.org/en/master/man/sphinx-build.html

[matthewfeickert]

@kratsg Once the -n is enabled then can fail on warnings for RTD also be turned on?

pyhf/.readthedocs.yml

Line 11 in 75f3cd3

#fail_on_warning: true

[kratsg]

@kratsg Once the -n is enabled then can fail on warnings for RTD also be turned on?

Nope, because epub has a different set of warnings we'd need to fix.