Deprecate `include` for all slide panels in decks

The existing implementation of "slides" within the curriculum introduces considerable noise to the Markdown content. With the PRs updates, the functionality and behavior remain the same and only affect the Markdown formatting itself.

All `{% capture slide %}` and `{% endcapture %}{% include slide-section %}` have been replaced with Markdown-only content and "slide indicators" using the `---` preceding and following any slide content.

Updated the [`README.md` (rendered)](https://github.com/github/training-kit/blob/no-more-includes-for-slides/README.md) to align with the changes and current curriculum format.