New API documentation

[mtrunkat]

I am opening this ticket to collect feedback on current documentation before we kick off the work on the new one.

Feedback from a power customer [Slack]:

He mentioned the need for better documentation, particularly at the Actor level, and clearer communication about any changes to data structures. He showed IBM API docs as an example.

[jancurn]

Points from me:

• support for OpenAPI specs for the api
• well integrated into https://docs.apify.com/
• provides code examples in our client libraries for Node.js, Python, and ideally also CLI