GHA depexts: some enhancements

[rjbou]

• spellcheck pass
• Add generate action script in spellcheck job
• cleaning, add some comments
• update local testing instructions
• add repository caching in docker build in order to lighten test script: there is ow 2 repositories, main one and one that only contains conf-* files for the tests in entrypoint
• make sure to use the good opam binary by expliciting full path

[kit-ty-kate]

Thanks!