Break up release workflow into separate workflows

[cjfullerton]

The release workflow contains several parts that could be separated into 'sub-workflows':

1. bump-version
2. build-for-release (separate from build-exes).
3. release-PyPI
4. release-GitHub
5. build-docs
6. update-website

This is assuming it's possible to call these in succession when a PR is merged into develop or main and to pass relevant information between workflows.

This could make modifying and testing the release workflow easier in future.

[cjfullerton]

After reverting PR on head branch, need to do something to source branch to create new PR. See this for some suggestions: https://medium.com/@shanikae/create-pull-request-from-a-reverted-git-branch-27219cadf9b5