this simplifies the setup a lot, and avoids weird looking `./file.md`
links showing up.
it also does not show regular URLs any more. currently the command
reference only has few of them, and not showing them in the offline
documentation is hopefully not a big deal.
instead of building more special-case solutions, clumsily preprocessing
the input, or issuing verbal rules on dealing with URLs, should better
be solved sustainably by not rendering relative links in `lowdown`:
https://github.com/kristapsdz/lowdown/issues/105
We upgrade to lowdown 0.8.0 [1] which contains a fix/improvement to a
behavior mentioned in this issue thread [2] where a big part of
lowdown's API would just call exit(1) on allocation errors since that
is a satisfying behavior for the lowdown binary.
Now lowdown_term_rndr returns 0 if an allocation error occurred which we
check for in libcmd/markdown.cc.
Also the extern "C" { } wrapper around lowdown.h has been removed as it
is not necessary.
[1]: 6ca7c855a0/versions.xml (L987-L1006)
[2]: https://github.com/kristapsdz/lowdown/issues/45#issuecomment-756681153
