Update documentation with wrapper plugin (was: Pluginifiy and rename default mode)

[n-osborne]

This will allow to release only the qcheck-stm plugin and the ortac-core translation.

Idea for renaming default mode is wrapper
We can also rename the ortac parckage into ortac-core

Update READMEs accordingly

[shym]

I reopen this issue, just to keep track of the fact that the READMEs still need to be updated.