Verify docs were built on RTD before creating a GitHub Release

[rmartin16]

Changes

• Introduces a dependency on RTD successfully building docs for the new v* tag for creating a GitHub Release.

Implementation

• When a new v* tag is pushed to the repo, GitHub initiates this repo's release workflow (and in turn the publish workflow as a consequence of a Release being "published").
• At the same time, presumably through a webhook between GitHub and RTD, the docs for the tag are built on RTD.
• So, this simply inserts a loop in the release workflow that confirms RTD successfully built the docs for the new version and allows the release process to progress.
  • This checks the built parameter of the payload returned from RTD's version detail endpoint which is defined as "the version has at least one successful build".
  • Since this is the first time RTD has seen the version, a value of true should imply the build that was just triggered completed successfully.
• As an example, consider the v0.3.14 release from April 11 @ 0311 UTC:
  • GitHub release workflow run
  • RTD build

Notes

• Depends on Introduce workflow to verify ReadTheDocs build .github#30.
• Ref: RTD is Showing Docs for v0.3.13 as stable After Release of v0.3.14 #1182

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

We can't really test this without cutting a release; but by inspection, it looks good. If it isn't, I guess I'll frantically fix it when we cut v0.3.15 :-)