add course-group, and relevant course-index, course-page contexts, templates, stylings

[possumbilities]

Description

• adds new course CCT, and allows hierarchical nesting to break things into Course > Unit > Chapter structures
• adds relevant backend fields to manage course specific data/content input
• utilizes a single .php template to dynamically display either course-index or course-page markup
• outputs an automated TOC listing, nested in accordance with how a particular course is setup on the backend (to avoid necessary duplication of efforts when a course is edited.)
• allows for a special embedded URL parameter, that when set to true reduces the page to a more limited layout to better embed within eLearning platforms as course material there.
• adds an attribution details areas to the bottom of pages, which can be filled in from a set field
• allows for future support of multiple courses
• dynamically creates previous and next links that will cycle through the chapter, then move up to the next unit sibling and continue
• hides courses from search, so they are not stumbled upon out of context (main pages featuring and linking to a course can still be searched of course. Avoids context collapse issues).

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

Developer Certificate of Origin

For the purposes of this DCO, "license" is equivalent to "license or public domain dedication," and "open source license" is equivalent to "open content license or public domain dedication."

Developer Certificate of Origin

Developer Certificate of Origin
Version 1.1

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Developer's Certificate of Origin 1.1

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    have the right to submit it under the open source license
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(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
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(c) The contribution was provided directly to me by some other
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