[cmake] Add man pages for the new executables

Summary: Both iguana and proof-manager-cli are lacking a target to build their man pages. Let's fix this so it can be added to our documentation on the website.

Test Plan:

cmake -GNinja .. -DBUILD_CHRONIK=ON -DBUILD_CHRONIK_PLUGINS=ON -DBUILD_PROOF_MANAGER_CLI=ON -DCMAKE_INSTALL_PREFIX=install -DBUILD_IGUANA=ON
ninja install-manpages-html

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18961