Replace unstable Zenodo-rendered badges with static Shields DOI badges

[Copilot]

Zenodo-hosted badge images intermittently fail to render on GitHub, causing inconsistent badge display (including in changelog entries). This PR switches DOI badge images to static Shields URLs while keeping DOI destination links unchanged.

• Badge source migration

  • Replaced all https://zenodo.org/badge/DOI/...svg image URLs with https://img.shields.io/badge/DOI-...-blue.
  • Applied consistently across:
    • README.md
    • doc/changes.md (all release badges)
    • .github/release-drafter.yml (release note template)
    • doc/maintenance.md (release process example)

• DOI consistency cleanup

  • Normalized the README badge DOI text to include the zenodo. segment (10.5281/zenodo.3781524) to match the target DOI link format.

• Example of the change

  [![Digital Object Identifier for PyGMT vX.Y.Z](https://zenodo.org/badge/DOI/10.5281/zenodo.<INSERT-DOI-HERE>.svg)](https://doi.org/10.5281/zenodo.<INSERT-DOI-HERE>)

  ⟶

  [![Digital Object Identifier for PyGMT vX.Y.Z](https://img.shields.io/badge/DOI-10.5281/zenodo.<INSERT-DOI-HERE>-blue)](https://doi.org/10.5281/zenodo.<INSERT-DOI-HERE>)

Preview

• https://github.com/GenericMappingTools/pygmt/blob/copilot/replace-zenodo-badges/doc/changes.md
• https://github.com/GenericMappingTools/pygmt/tree/copilot/replace-zenodo-badges#pygmt