Add explainer for v2 (arbitrary content)

[beccahughes]

I wrote an explainer for the arbitrary content based on @mounirlamouri's proposal document. I was thinking we should keep all the arbitrary content bits on a separate v2 branch so we can iterate on it easily and fix issues and then when are ready we can merge it into master.

@beaufortfrancois @mounirlamouri WDYT?

[mounirlamouri]

It might be good to more clearly state goals and non-goals as explained in https://github.com/w3ctag/w3ctag.github.io/blob/master/explainers.md

Some recommend to drop IDL in explainers. I do not have a strong opinion but I do not think we should go with the Document and Element level API. Dropping the IDL from the explainer will punt that discussion ;)

[beccahughes]

Okay done

[beccahughes]

@beaufortfrancois @mounirlamouri do you have anything else you would like to add or can I merge this?

[beaufortfrancois]

Except from the nit about window.open, I'm good.

[beccahughes]

Thanks!