docs: a lot of restructuring, and YAML focusing

[sbalnojan]

This is a draft PR, sadly I wasn't able to finish it. It does contain:

• a new index page (a Meltano at a Glance page) <= @DouweM needs to take a look at and help to rewrite (it's just a draft with the key sections!)
• a rewritten "connect to data" part 1 that is now YAML-first. (Wasn't able to rewrite the other ones so far)
• A skeleton for all other parts yet to be written (sorry!!!)

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[WillDaSilva]

@sbalnojan Do you think we'll ultimately merge this (no rush if so), or should it be closed?