docs/xenopsd: List the child pages using the children shortcode

[bernhardkaindl]

The docs commonly use the children shortcode to list child pages,
we may use it for the xenopsd walk-throughs and designs as well.

[bernhardkaindl]

JFYI, Here is a preview of the docs with this PR:

https://bernhard-xapi-onrender-com-pr-8.onrender.com/xenopsd/walkthroughs/index.html
https://bernhard-xapi-onrender-com-pr-8.onrender.com/xenopsd/design/index.html

PS:

How this works:

I configured a fork of this repository for automatic Hugo previews on http://render.com using this blueprint: https://github.com/bernhard-xapi/bernhard-xapi.onrender.com/blob/main/render.yaml. After enabling previews for pull requests , I got this preview site created automatically after opening a local PR in the repository. This is the PR showing the creation of the preview site: bernhard-xapi#8 (it only supports local pull requests within the repository, so I had to create a local PR for it)