Add new getting started page

[luabud]

For #16678

[luabud]

@kimadeline here's the new PR 😅

[digitarald]

@luabud a few of the GIFs ballooned in size of multiple Mb.

I tried ImageOptim and it reduced the 13 Mb new_notebook.gif to 3.9 Mb – still a lot. Would be worth to cut them down further and compress.

[luabud]

@digitarald thank you so much for looking into it! I agree anything the sizes were too large, I changed many of the GIFs to be just screenshots to solve that (FYI @jmew). Now we have only 3 GIFs that altogether sum up to 1Mb, but I think we'll want to migrate to animations like you all did, so it should be a temporary thing.

[digitarald]

Looks great. I was also hoping for the smoothly animated product video look, but sadly crisp loops just get too big. I want to eventually try lightweight SVG animations, which can also add some delight.