Deploy docs to GitHub Pages

[arnaucasau]

Summary

This PR changes the deploy-docs.yml workflow to deploy the documentation to GitHub Pages to migrate the project away from the qiskit.org domain.

The deployment to the gh-pages branch will not overwrite the locale folder, where I will save all the translations in a follow-up once the first deployment is done.

More information about the change and a quick guide to set up GitHub pages can be found at: Qiskit/ecosystem#578

Reminder task for maintainers here to update the doc link in the About text for the repo on the main page once this change happens and the docs are re-deployed.

• Update About text doc link

[coveralls]

Pull Request Test Coverage Report for Build 7615325851

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 75.822%

---

Totals
Change from base Build 6925373552:	0.0%
Covered Lines:	646
Relevant Lines:	852

---

💛 - Coveralls

[woodsp-ibm]

@arnaucasau With the deploy translatable strings script deleted I imagine the gettext section of the tox.ini file can be removed (the script used tox to do the strings). Also there is gettext config in the conf.py such as gettext_compact that is not really needed.

While you are having locales folder there not cleaned during deploy it seems that any further updates of the translations are not going to happen - hence the removal of the script.

I figured to sort things out here, since the other apps (Nature, ML and Opt) should be pretty much identical in regards this change. Once it all seems fine then backport this PR and deploy the docs to make sure its all as expected.

[arnaucasau]

Thank you @woodsp-ibm for your review! In the last commit, I removed the gettext section from the tox.ini and the config related to it that I found in conf.py (I think that's everything).

I'll make the same changes to Nature, Machine learning, and Optimization 👍

[woodsp-ibm]

@arnaucasau Thanks for the changes. I labelled this so it will be backported to stable after which I will deploy the docs again so things can checked out as/if needed.