[alexander-schranz]

• Build 3.x from sulu 3.1 and 2.x from sulu 2.6, version switcher lists only these two
• Replace docs/2.0 to 2.6, 3.0, 3.1 and latest with a redirect to 2.x or 3.x, the URL fragment is kept
• STABLE_VERSION is 2.x, README maintenance steps updated
• Related: Fix styleguide example errors and check examples in CI sulu#9234 (fixes errors in the 2.6 examples, the 2.x docs show them)
• Check: after merge the push-to-master run creates docs/2.x and docs/3.x, then jsdocs.sulu.io/2.6/#!/List redirects to /2.x/#!/List