Changing default value of the convergence residue in iterative solvers to zero. Small positive value can spoil solution of ODE solvers.

- removed outdated link to the "stable version"
- described other installation methods: AUR package and git submodule
- updated description of the install script and its targets

Doxygen changed their way of naming HTML files converted from Markdown
in the 1.9.3 release, but linking to other Markdown files is now
supported natively. No need for links to .html and the \page macro at
the top of each Markdown page.

The example should be the same for both .cpp and .cu, otherwise the
output might be inconsistent (e.g. when .cpp is included, but .cu is
compiled and its output is used).