Fix pages build and add BUILD_SDK variable

Pages build did not work as expected with PAGES_BUILD_REF. So the 'only' keyword is exchanged by an rule. The default rule is now to build the docs only for the default branch. PAGES_BUILD_REF is now obsoleted and removed.

Also it is now possible to build the SDKs automatically by setting the BUILD_SDK image name.

This change should be compatible and shouldn't break existing things, besides it changes the behaviour of building the docs branch (but the new behaviour seems very sensible for me)