chore: Switch from edx-sphinx-theme to sphinx-book-theme

[xitij2000]

The edx-sphinx theme is being deprecated, and replaced with sphinx-book-theme. This removes references to the deprecated theme and replaces them with the new standard theme for the platform.

Testing instructions:

• Run make docs and see that the docs are generated properly and use the sphinx-book-theme

See https://github.com/openedx/public-engineering/issues/200

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[xitij2000]

@feanil This repo seems to be using Python 2 and seems to be hosting its own copy of edx_theme. It is using Sphinx 1.5.2 while the current version is 6, and doesn't seem straightforward to upgrade.

Is this supposed to be archived? If not I think we'll need to have the docs built by Python 3 while the rest of the repo uses Python 2.

[feanil]

@bmtcril do you know the answers to @xitij2000 question? Is this repo something that should be archived?

[bmtcril]

My understanding is that 2U is maintaining this for at least their own pipelines, but I'm not sure at what level. The amazing @HassanJaveed84 is the last 2Uer that seems to have touched it so may have more details, but I don't think it's ready to be archived yet.

[xitij2000]

@feanil @bmtcril Sure. I think that clarifies things. I can see two options in the scope of the edx-sphinx-theme deprecation task then:

1. We can ignore this repo for now. It doesn't pull edx-sphinx-theme as a dependency, rather includes its own copy, so it should not impact its archival.
2. We can upgrade the makefiles etc here to build on Python 3 for docs while the rest of the repo runs on Python 2.

[bmtcril]

I'm on board for option 1 given the state of things

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