API Streamer docs improvements

[Srushtika]

This issue contains things to discuss, consider or update in the existing version of the API Streamer docs.

Consumer page

• direct invitation should link to something discussing how to receive/send direct invitations once we have such a resource.
• log into the API Streamer's consumer dashboard to link to API Streamer consumer dashboard.

Overview page

• Why use the API Streamer to build realtime APIs for your streaming data? to signify 'What API Streamer is for`?
• A few links to prove the claims in the overview section.

Producer page

• Definitely need some links and screenshots in all sections, for example, how to create things, selecting apps, etc.

Hub page

• Is it ok to use 'host' to describe making one's data streams available on the Hub?
• Make the copy in the page self-sufficient

General

• Move all the guide-like explanation into support articles and link them from docs.

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