doc: Make build system insert version in Doxyfile

Summary:
Backport of https://github.com/bitcoin/bitcoin/pull/10155/commits/168a7034f5eefcf657f6332f0b7966e439ca4df5

Completes T645

Test Plan:

make check
ninja
less <build>/doc/Doxyfile # verify it exists and is generated with the correct version number

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3508