Set version of release branch to 3.1.0.
Replace download links by link to GitHub releases page.
Add generated PDF manual of v3.1.
#365 Remove git checkout release command from Quick Start script so it uses the master branch.
Merge remote-tracking branch 'github/release' into develop
Revert "Set version of release branch to 3.1.0."

This reverts commit 60ef43581fd66dfa7dc4dacd693d51605d070bc0.