Changes

Enable the CMake based doc generation and installation. (#1239)

  - It is now possible to build the Doxygen and Sphinx documentations with CMake. There are multiple build and install targets that can be used to give more flexibility.

    `doc-doxygen-html`: Geneate the doxygen based html documentation.
    `install-doxygen-docs`: Install the doxygen based html documentation.

    `doc-sphinx-latex`: Generate the UsersGuide latex files with Sphinx.
    `doc-sphinx-latex-pdf`: Generate the UsersGuide pdf from the latex files.
    `install-sphinx-pdf-docs`: Install the UsersGuide pdfs (just one right now).

    `doc-sphinx-html`: Generate the UserGuide html files with sphinx.
    `install-sphinx-html-docs`: Install the UserGuide html files.

    `install-sphinx-docs`: Install both the html and pdf UserGuide files.

    `install-docs`: Install all documentation files. Doxygen html, Sphinx UsersGuide html, and Sphinx UsersGuide pdf.

  - These targets are created according to the availability of Doxygen and Sphinx on the system. If neither is found, there will be no `install-docs` target at all.