Improve instructions describing how to cite PISM

[ckhroulev]

• Add a section in the Sphinx docs describing how to cite PISM (both research papers and software). This section can use Sphinx to include the BibTeX file with a list of research papers to cite.
• Replace ACKNOWLEDGE.rst with an automatically-generated file that duplicates this section, but contains BibTeX entries in it. I think this duplication is necessary because a user who downloaded PISM's source code should be able to read these instructions without rebuilding PISM's Sphinx docs.
• Update https://www.pism.io/citing/ so it contains a link to the new section in Sphinx docs instead of repeating ACKNOWLEDGE.rst