Allow Linux CI to push changes to forks

[ZanSara]

Right now, the documentation and code style bot will fail on forks as the checkout@v2 action fails. This leads to the situation in which no code style or documentation will be updated on the PR and neither on merge to master (the bot can't push to master right now), but they will trigger on the first PR opened from the updated master.

This PR tries to make checkout@v2 push changes back to the source fork, if possible.

[ZanSara]

Summary of the solution I've found:

1. It seems to be not possible by design, for a bot that runs on the original repo, to push changes back to a contributor's fork.

2. CI workflow triggers, however, can trigger on either the original repo and the forked one: specifically, push runs on the fork (which sees the push event) and pull_request works on the original (which receives the PR request).

3. By using push as a trigger, the action always run with a token that has permission to commit back (it stays local, in a way).

4. I made "Linux CI" work on pull_request and "Code & Documentation Updates" work on push

5. Gotchas:

   • Contributors have full control on the actions that run on their side and can disable them completely
   • We have limited visibility of what's happening on their end
   • We will not see the status of their actions on our side, we have to trust they run them
   • On the other hand, if they don't run the actions, the only issue would be to have some "spurious" Black and docs changes on other, unrelated PRs, which might slightly conflict one with the other. Annoying but not critical.

We could implement a "Verify Code & Documentation Updates" action on our end which does not try to commit back the changes, but will always fail if it triggers: that's good because on our end such action would trigger only if the the contributor has not run the corresponding action on their end.

@julian-risch I am in the process of implementing the last "verification" action, feel free to start reviewing before and let me know if you see something that needs change.

[julian-risch]

Looks good to me so far. Just add a brief description to the contributor guidelines: https://github.com/deepset-ai/haystack/blob/master/CONTRIBUTING.md please. Maybe even with a screenshot of what the contributor needs to do to run the GitHub action?
Let's test the process ourselves with an account that is not in the deepset GitHub organization yet (once your changes are merged).