ci/cd: docs workflow doesn't work properly for non-custom github pages domain

[Kamil Samigullin]

The default host looks like <owner>.github.io/<repo>, but pages contain href="/_next/static/css/...", so it tries to load the source from <owner>.github.io/_next, not <owner>.github.io/<repo>/_next.

[Kamil Samigullin]

• quick fix withsparkle/service@1c9c9a9

[Kamil Samigullin]

finally

• https://github.com/withsparkle/service/blob/f9aee3527f02981bbb69aa688955ed495ae4bad1/.github/workflows/cd.docs.yml#L48-L57
• https://github.com/withsparkle/service/blob/f9aee3527f02981bbb69aa688955ed495ae4bad1/.github/workflows/ci.yml#L94-L100