[CMAKE] Flag the HTML man pages as generated

Summary:
This prevent them from being shown by default during reviews.
Follow-up for feedback from D7252.

Test Plan:

ninja install-manpages-html

Check the @generated marker is present in the files

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7255