Continuously update public documentation

Business justification: Improved developer experience. No need to wait for release cycles. 

Effort estimate: 

Description:

User story: As a developer using Qt, I want the public documentation to always be as current as possible.

Currently, documentation is only updated upon release, but we also publish documentation snapshots. It should be possible to update the official public documentation for our various releases continuously, that is, when we fix a documentation-only issue, there is no reason for it to have to wait for the next release.

Cases where documentation follows bug fixes could be handled by either using separate branches or a note specifying which (yet unreleased) version the documentation refers to.

For quality, we are already keeping records through Gerrit on what went in and when so we always know when we updated what.