[CMAKE] Wrap documentation installation into a function

Summary:
This simplifies it a bit and makes it easier to expand. There is no
change in behavior.

Test Plan:

ninja install

Check the docs are installed.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7224