Docs: Doxygen Documentation Deployment

[jinzx10]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

What's changed?

A new workflow that deploys the doxygen-generated html documentation as github pages is added. The workflow will be triggered whenever there is a push event (like the merge of a PR) to the "develop" branch.

Update:
The workflow is configured such that only one deployment can exist at any time, i.e., if multiple PRs are merged within a short period of time, only the one triggered by the last merge will be executed; those triggered by previous PRS will be cancelled.

A preview of the documentation can be found at https://jinzx10.github.io/abacus-develop/

[mohanchen]

Nice feature.

[jinzx10]

The workflow is configured such that only one can exist at any time. If multiple PRs are merged within a short period of time, while each of them will trigger the deployment action, only the one triggered by the last merge will be fully executed; those by the previous PRs will be automatically cancelled, even in progress.