This moves auto-generation of Markdown documentation out of
`contrib/devtools/gen-manpages.sh` and into the build system, with a new
ninja target `doc-md`. `contrib/devtools/gen-manpages.sh` returns to its
original purpose of generating manpages only (can be integrated into the
build system later as well).

The auto-generated Markdown documentation is no longer committed into
the repository, but built as needed. CI will build it for GitLab pages,
so that docs.bitcoincashnode.org will always be fully up-to-date with
the master branch. A `DOC_ONLINE` option has been added to the build
system to differentiate between documentation generated for the website
and for local use (default off), replacing the previous CI hacks.

Test plan: `ninja doc-html` (with `-DDOC_ONLINE=ON` and
`-DDOC_ONLINE=OFF`),
https://bigblockiftrue.gitlab.io/bitcoin-cash-node/