build: remove sphinx-book-theme constraint to use latest release

[mariajgrimaldi]

Description

This PR removes the existing sphinx-book-theme to use the latest release. Here's the comment we realized we need to utilize the constraint: https://github.com/openedx/openedx-events/pull/169#discussion_r1073980224 Also, we updated the theme configurations for the latest suggested on the open edx docs: https://docs.openedx.org/en/latest/developers/how-tos/add-sphinx-docs-to-a-repo.html.

[openedx-webhooks]

Thanks for the pull request, @mariajgrimaldi! Please note that it may take us up to several weeks or months to complete a review and merge your PR.

Feel free to add as much of the following information to the ticket as you can:

• supporting documentation
• Open edX discussion forum threads
• timeline information ("this must be merged by XX date", and why that is)
• partner information ("this is a course on edx.org")
• any other information that can help Product understand the context for the PR

All technical communication about the code itself will be done via the GitHub pull request interface. As a reminder, our process documentation is here.

Please let us know once your PR is ready for our review and all tests are green.

[openedx-webhooks]

@mariajgrimaldi 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.