[FLINK-17239][docs] Streaming Analytics tutorial

[alpinegizmo]

What is the purpose of the change

This tutorial introduces event time, watermarks, and windowing, in a somewhat basic way. concepts/timely-stream-processing needs to be reworked to fit better with this content, which is in FLINK-17247.

Brief change log

Adds a new tutorial and its figure.

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Last check on commit 5cdc506 (Tue Apr 21 11:56:54 UTC 2020)

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[NicoK]

very nice! +1 after addressing a few comments

[NicoK]

I think, I would prefer to have this as a figure, or at least increase the font size and center it?

[alpinegizmo]

I made it more presentable. (But I confess that the html I used isn't very pretty.)

[NicoK]

yes, I would agree on that ;)

let me have a look at improving the html a bit...

[NicoK]

this seems to look nice (I'll apply it to your PR unless you object):

<div class="text-center" style="font-size: x-large; word-spacing: 0.5em; margin: 1em 0em;">
··· 23 19 22 24 21 14 17 13 12 15 9 11 7 2 4 →
</div>

[NicoK]

lgtm

I'll rework the html a bit and will then merge