There's a merged pull request on the void-packages repository

gtk4: update to 4.2.1, split docs, gi-docgen
https://github.com/void-linux/void-packages/pull/31820

Description:
<!-- Mark items with [x] where applicable -->

#### General
- [ ] This is a new package and it conforms to the [quality requirements](https://github.com/void-linux/void-packages/blob/master/Manual.md#quality-requirements)

#### Have the results of the proposed changes been tested?
- [x] I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
- [ ] I generally don't use the affected packages but briefly tested this PR

The gi-docgen package and its dependencies are taken from the libadwaita PR (#31102), I will remove them from there when this PR is merged.

I split the documentation because it's quite big - 70 MB, the -devel subpackage is 10 MB (mostly gir files).