Part of #1931

Changes need to happen on the branch pmd7 in https://github.com/pmd/pmd.github.io.

Enable automatic deployment to https://pmd-code.org/pmd7/ for preview via travis (jekyll build, scp, ...).
Exclude "pmd-*" directories, the docs will be hosted on docs.pmd-code.org. This makes the landing page manageable.

Adjust the coloring of the landing page. Also favicon.