Part of #1931

The preview location is https://docs.pmd-code.org/pmd-doc-7.0.0-SNAPSHOT/.

Changes need to happen on the branch pmd/7.0.x
in the main repo https://github.com/pmd/pmd.

See path docs/images/logo.

Also adjust the coloring of the documentation: color theme should use green instead of blue (heading, primary button color, ...).

Also add the logo to README.md.