Add a workflow to upload baseline images as a release asset

[seisman]

Description of proposed changes

In this PR, a workflow is added to upload all baseline PNG images as a release
asset when a release is published.

Fixes #1201.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

Because the workflow only runs when a new release is made, I committed it directly to my own fork (https://github.com/seisman/pygmt/blob/master/.github/workflows/release-baseline-images.yml), and made a release at https://github.com/seisman/pygmt/releases/tag/v0.3.2.

The workflow runs well (https://github.com/seisman/pygmt/actions/workflows/release-baseline-images.yml) and the baseline images are correctly uploaded at https://github.com/seisman/pygmt/releases/tag/v0.3.2.

[weiji14]

Cool, great work @seisman! This new workflow needs to be documented in maintenance.md. Also, do we want these baseline images stored on Zenodo too? If so, we'll need to update the PyGMT 'how to release' documentation too.

[seisman]

This new workflow needs to be documented in maintenance.md.

Done in dc30267.

Also, do we want these baseline images stored on Zenodo too? If so, we'll need to update the PyGMT 'how to release' documentation too.

Does it mean we (mainly you) have to manually upload the zip archive to Zenodo?

[maxrjones]

Looks good to me.

[weiji14]

Also, do we want these baseline images stored on Zenodo too? If so, we'll need to update the PyGMT 'how to release' documentation too.

Does it mean we (mainly you) have to manually upload the zip archive to Zenodo?

Yes, I'll need to upload it to Zenodo. Maybe also change:

1. the note at https://github.com/GenericMappingTools/pygmt/blob/master/doc/maintenance.md#archiving-on-zenodo to say "Grab both the source code and baseline images zip files from the GitHub release page and upload them to Zenodo using the previously reserved DOI".
2. The last 'Release' bullet point at https://github.com/GenericMappingTools/pygmt/blob/master/.github/ISSUE_TEMPLATE/release_checklist.md to say "Manually upload the pygmt-vX.Y.Z.zip and baseline-images.zip files to ..."