ci|docs: automatic docs deployment when merging into develop

[romanc]

Description

This PR adds the automatic deployment of documentation from the docs/ folder to https://noaa-gfdl.github.io/NDSL with every merge into develop.

@fmalatino: we might need configuration changes (in the repo settings) for this to work. It should be the same as for PyFV3, where we already automatically deploy docs to https://noaa-gfdl.github.io/PyFV3/.

@FlorianDeconinck this will deploy the whole contents of docs/, not just the docstrings. I remember a discussion to bring some of the contents from SMT (e.g. the tutorials) over here before we publish docs. Should we do a pass at the docs/ folder first, before we deploy docs?

How has this been tested?

With #268 merged, we at know that the docs will build. The deployment is untested. However, it's a standard building block and I just copied the workflow from PyFV3 where it already works. If anything fails, then it's likely a configuration issue (and I don't have access rights to check if anything needs to be configured or if things are okay).

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
  N/A

[FlorianDeconinck]

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