To help with man-db development, you'll probably want to start by checking out the latest code from revision control:
git clone https://gitlab.com/cjwatson/man-db
Then run ./autogen.sh before starting. You'll need
Autoconf,
Automake, and
Gettext; it is usually
best to install your distribution's packages of these if available. You
may also want to have a copy of
Gnulib handy.
You should read the docs/HACKING.md file.
You can also just browse the source code using a web browser.
There is a mailing list for developers, although it is as yet rather quiet.