#9 0.3.1: republish under the public maintainer address

Version bump only (plus the two docs that name the version). 0.3.0 and earlier npm versions recorded a private email in their publish metadata; 0.3.1 republishes the same code under ei@nsisong.com so the old versions can be withdrawn.

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Summary by cubic

Republishes the 0.3.0 code as 0.3.1 under the public maintainer address ei@nsisong.com, so the 0.3.0 and earlier npm versions that recorded a private email in their publish metadata can be withdrawn. Also updates the version references in the changelog, guides, and workflow docs. Projects pinned to 0.3.0 should move to 0.3.1 (POSTLINE_VERSION and uses: 1cbyc/postline@v0.3.1 in the workflow).