Various improvements to documentation (#7283)

Various improvements to documentation (#7283)

This PR adds a couple of improvements to our documentation setup
- add an explicit dependency on the extension so that sphinx rebuilds pages appropriately when an extension is modified
- sphinx now emits warnings for examples that are not implemented in all languages (smt2, cpp, java and python)
- add -v to make the sphinx output more log-file friendly
- avoid a warning when the java bindings (and thus java docs) are not build
- replace file(WRITE by configure_file to avoid rather common erroneous rebuilds of python docs
- fix a reference to a label in the python docs
- suppress timestamps in javadoc output to avoid docs-ci being triggered for every PR
- improve placeholder message when java bindings are disabled