xilem: Docs updating

[ArtyomSinyugin]

Draft
This PR is the proposal for xilem docs starting page. But I think I will be able to make some more commits to improve documentation.

[DJMcNab]

Thanks for providing a foundation here. The CI needs to be green to land this.

[ArtyomSinyugin]

Thanks for providing a foundation here. The CI needs to be green to land this.

Thank you for comments. Before I left some red alerts for purpose: I want to do some more work inside this PR. So, now I am going to look at Masonry docs and then will make some inserts and updates.

[DJMcNab]

Thanks for doing this - the content for the views look good.

As mentioned, the new README content needs to be added to the "formatting" CI job.

I'm willing to make the corresponding changes to the README myself, outside of this PR, if you prefer.

[DJMcNab]

I've converted to draft based on your Draft in the top-level body of the PR. When you're ready for this to be reviewed/land, please remove it from draft (or one of us can finish it off if you need)

[ArtyomSinyugin]

I've converted to draft based on your Draft in the top-level body of the PR. When you're ready for this to be reviewed/land, please remove it from draft (or one of us can finish it off if you need)

I think I've finished.
I would like to describe in docs some more elements, but for image, portal, progress_bar, prose, spinner, task`, etc. it take much more time for me to figure out how these elements work.

Maybe I'll do the rest later, maybe in the next PR (if nobody does it in the near future).