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, replaces them with the new
standard theme for the platform, and updates the theme configuraiton to use the
new theme.

Testing instructions:

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

See openedx/public-engineering#200

[openedx-webhooks]

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[Cup0fCoffee]

@xitij2000 Are the changes kept to the minimal because the repo has been using the theme before and the settings that had been set are assumed to be the desired ones? Just asking, since I've notice a minor difference to how the docs in the other repos look after the change - no light/dark mode theme button, no changes to the footer, and perhaps there is more. Not a blocker on my side.

[Cup0fCoffee]

👍

• I tested this: make docs
• I read through the code

[xitij2000]

@xitij2000 Are the changes kept to the minimal because the repo has been using the theme before and the settings that had been set are assumed to be the desired ones? Just asking, since I've notice a minor difference to how the docs in the other repos look after the change - no light/dark mode theme button, no changes to the footer, and perhaps there is more. Not a blocker on my side.

Yes, the rest of the config was already in place.

[mariajgrimaldi]

Thank you for the changes!

[openedx-webhooks]

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