Release validation for `v4.0`

## Website/Markdown settings

* [ ] Add `.markdownlint.yaml` file
* [ ] move fonts to the root folder and symlink them from everywhere else
* [ ] explore whether the same can be done with the assets
* [ ] split images into a root folder with common images and one for the specific versions for the rendered SVGs

## Examples

* [ ] Rework icedelivery
* [ ] Explain lifetime semantics of chunks/samples and user restrictions: loan needs to be followed with publish or release, MAX_CHUNKS_IN_USE on publisher and subscriber side etc., API is not thread-safe, update asciinema record (@MatthiasKillat)

## New articles

* [ ] How to write integration tests
* [ ] Performance results section

## Last tasks

Start with the Website/Markdown settings checklist and the `icedelivery` example. Review the requested `.markdownlint.yaml`, font and asset layout, versioned SVG images, lifetime-semantics explanation, and asciinema update, then check the planned integration-test and performance-result articles. Done means the listed release-validation tasks are completed and checked off.

Written by the indexing model from the issue text.