meson: Detect and reject invalid rst2html5 command
meson: Detect and reject invalid rst2html5 command

The version coming from the rst2html5 package instead of the
docutils package is unable to successfully generate the libvirt
documentation.

Examples of users encountering build issues because of the wrong
version of rst2html5 being installed on their systems:

  https://gitlab.com/libvirt/libvirt/-/issues/40
  https://gitlab.com/libvirt/libvirt/-/issues/139
  https://gitlab.com/libvirt/libvirt/-/issues/169
  https://gitlab.com/libvirt/libvirt/-/issues/195

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

jfehlig/libvirt,jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,zippy2/libvirt,nertpinx/libvirt,nertpinx/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,olafhering/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,olafhering/libvirt