remove/update "External Libraries" page

Te page about installation instructions for external libraries https://www.dune-project.org/doc/external-libraries/ is totally outdated and simply contains wrong information. This should be fixed either by

• removing this page. It is not necessary to include information of it is also available directly from the external library webpage. Those should be linked somewhere, e.g. directly in the cmake package information
• Update this page. Include links to all external libraries currently supported in the core modules. Maybe add special installation instructions or comments how to use with dune.

I would vote for removing this page, since it is a lot of work to have this up-to-date.