Give some prominence to the Community section

[matthiasrohmer]

Currently the community section falls a little short in comparison to the About section although it holds very central information to the AMP Project with the Roadmap and Working Group pages.

Therefore the idea is to also add icons and short descriptions to the community navigation items as well. Also in preparation of adding the new AMP for Wordpress landing page to the community section as well.

/cc @sebastianbenz for thoughts