Update small issues in documentation

Some things in the documentation to update for easy first pull request. Feel free to post more as you find them:

[ ] doc/USER_MANUAL/bug_reports.tex links to the CIG mailing list, but should instead link to github issues (https://github.com/geodynamics/specfem3d/issues/new)
[x] AUTHORS file in the main directory should include a link to the actual authors file doc/USER_MANUAL/authors.tex, and the online version https://specfem3d.readthedocs.io/en/latest/authors/
[x] A common place to keep instructions for how to contribute to a software package is a file called CONTRIBUTING.md. SPECFEM3D instead uses this wiki page: https://github.com/geodynamics/specfem3d/wiki. We should have a file called CONTRIBUTING.md that simply says to check that wiki page instead.
[x] The file specfem3d/doc/how_to/HOWTO_configure_the_GPU_version.txt is not needed anymore, as GPU configuration is described in the manual in https://specfem3d.readthedocs.io/en/latest/02_getting_started/#using-the-gpu-version-of-the-code
[ ] The content of the file specfem3d/doc/how_to/README_gravity_perturbation.txt should be added to specfem3d/doc/USER_MANUAL/10_gravity_calculations.tex and afterwards deleted. This requires some conversion from plain text to latex.

SPECFEM / specfem3d