Misc. improvements to docs build system

Misc. improvements to docs build system

- Get rid of GMX_BUILD_WEBPAGE.  It did not disable any user-visible
  detection, but only affected what build rules were generated, and none
  of those affected the default 'all' target.  Now the webpage target is
  always created, and any informative messages that were previously
  given during CMake time with GMX_BUILD_WEBPAGE=ON are now given when
  the user tries to build the target.
- For all the documentation custom targets, create a dummy target that
  gives a message to the user when building if the tools required to
  build that target are not available.
- Reduce the required dependencies for the 'webpage' target.  If some
  non-essential parts cannot be built, just give a message to the user
  and build the remaining parts with some broken links.

These should make the build system easier to document and to understand
from the user's perspective.

Change-Id: Ib65e3348b88b1e0461f1f69ba1218f09d376a6dc