ci: test the code examples within docs page

[masci]

Is your feature request related to a problem? Please describe.
Code examples from https://docs.haystack.deepset.ai/ can be easily copied and pasted into a notebook for testing, which is great. Sometimes though, the code doesn't work and it might be hard to find out why if you're a fresh Haystack user.

Describe the solution you'd like
We should routinely test all the code snippets from the docs pages; docs are edited on Readme but we don't need immediate feedback, it could be enough running the tests once per day in a nightly fashion.

Describe alternatives you've considered
Alternatives look overly complex and I wouldn't put them here.