Add disk io stats to EWW_DISK

[bkueng]

Very cool project.

Description

This updates system_stats and exposes read and written to EWW_DISK, allowing to monitor disk io stats.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing

[w-lfchen]

this only works after running cargo update --recursive sysinfo, which alters the lockfile. otherwise, there are dependency resolution errors.

[w-lfchen]

this should leverage the power of the type system.
consider this, from #1217:

Suggested change

	c.temperature().unwrap_or(NAN).to_string().replace("NaN", "\"null\""),
	c.temperature().map_or_else(|| String::from("\"null\""), |temp| temp.to_string()),

[w-lfchen]

it might be unclear which unit this is. consider documenting it

[w-lfchen]

cleanup, nothing big

Suggested change

	let (ref mut last_refresh, ref mut disks) = &mut *DISKS.lock().unwrap();
	disks.refresh_specifics(true, DiskRefreshKind::everything());
	let (ref mut last_refresh, ref mut disks) = *DISKS.lock().unwrap();
	disks.refresh(true);

[bkueng]

Thanks for the review, I addressed your comments.

[w-lfchen]

looks good now, thank you! <3
(only did some rough testing but should be fine)