Ship devhelp docs in tarball
Use a dist script to generate those docs and include them in the
tarball, the meson build definition has been changed to also account
for this shortcut if building from tarballs.
This avoids the hotdoc dependency and its dependencies (e.g. npm
modules) when building from tarballs.
Tarball size increased ~400KB, which stands on the acceptable size
since the tarball is still less than 2MB.