Complete versioned documentation generation process for dynamic pages since 0.11.0-M8

[lefou]

Until version 0.11.0-M8 we had documentation for each released version. Then, @lihaoyi extended the documentation to include executable examples, which we also run as part of our integration test suite. Unfortunately, that means, just checking out tags to generate versioned documentation isn't enough anymore, as we generate some files as part of the build process.

To fix this we need to preserve the generated files and commit them into some dedicated branch(es), so we can re-use then when we generate the documentation page with antora.