Tutorial improvements

[rkratky]

Description

This PR revamps the Netplan tutorial:

• Some reorg for better flow.
• Extensive language and formatting improvements.
• Moves parts to 'how-to guides' where appropriate.
• Tutorial landing page improved.

In addition:

• How-to guides landing page slightly improved in structure (to be revisited).
• Minor lang. + mark-up fixes in YAML reference.

Fixes FR-2811.

[slyon]

Thank you very much, Robert!

I very much like the breaking down of the big tutorial into smaller chunks and the overall structural improvements.

I wonder if the new how-to guides should be linked from the bottom of the tutorial, e.g. in a "Future reading" section or so?

I've left a few inline comments of broken links and one logical question wrt IPv6 addresses which shouldn't be there.. Those should be fixed before merging.

@daniloegea Do you have anything to add to this, as it was your document intially?

[daniloegea]

Thanks, Robert. I looks good. I just found a typo you might want to fix.

[slyon]

Thanks for fixing the remarks! LGTM

note: I pushed a tiny fix for whitespace formatting in the CLI output.