Add v0.3.1 release notes for the monospace console restyle

[tamnd]

Adds the docs/ release-notes page for v0.3.1, the patch that restyles ant serve into a flat monospace terminal (merged in #6).

• New page docs/content/release-notes/v0.3.1.md, weight: 5 so it sorts newest-first above v0.3.0 (weight 10) and v0.2.0 (weight 20).
• Describes the visual change only: one embedded Geist Mono face throughout, square corners everywhere, flat panels that separate by tone rather than borders or shadows. Notes that the CLI, JSON envelope, read path, and terminal console are unchanged from v0.3.0.

Ships with the v0.3.1 tag so GoReleaser publishes the matching binaries, packages, and ghcr.io/tamnd/ant:0.3.1 image.