Added "Getting started" page related to #87

[ariguiba]

No description provided.

[fonsp]

Nice! I'm happy that we are adding a Getting Started page :)

Could you still:

• Link to other pages about markdown, plots, etc to show what you can do with Pluto. In Step 4, we now say "Start Coding" -> "Pluto offers a lot of features" -> "Delete a cell" which doesn't make sense.
• Maybe we can remove the begin-end docs since Pluto will explain it automatically if needed? I would like to talk more about what you can do, instead of what you can't do. Also it's not one line of code, but one expression.
• Can you add alt-text for all images?

[fonsp]

Can you link PRs to Issues? https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue