amend! Revert "docs: inline document-features usage, remove dep."

Revert "docs: inline `document-features` usage, remove dep."

This reverts commit 3d5bec6, with an
update to `document-features`, and preferring to keep new `feature`
content. To be clear, the only difference I have observed is the
addition of the `serde` feature.

In case it shortens anyone's search, the specific issue resolved is
[`slint-ui/document-features`#20](slint-ui/document-features#20).

Author: ErichDonGubler