content editing update (#439)

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## Summary by CodeRabbit

* **Documentation**
* Major editorial and structural overhaul of the instructor authoring
guide for clarity and consistency.
* Revised headings/styles and reorganized content to foreground Markdown
usage and editing workflow (section selection, renaming, hotkeys).
* Reworked Custom CSS guidance with clearer options and course-level
steps.
* Expanded Markdown editor instructions, formatting reference (headers,
lists, code blocks), code-copy behavior, math/LaTeX, collapsibles,
callouts, and media embedding.
  * Updated examples, images, and interaction notes.

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Author: LolaValente