Update to markdown.instructions.md and addition of GFM and generic markdown instructions - Fixes #1108 (#1109)

* CHANGE: Update markdown instructions and add new files

- Added new markdown content creation guidelines.
- Introduced GitHub Flavored Markdown (GFM) instructions.
- Updated CommonMark Markdown instructions with detailed rules.
- Removed redundant content from the original markdown instructions.

* CHANGE: Update markdown content guidelines

- Refine code block formatting instructions in markdown content creation guidelines.
- Add detailed HTML block types for GitHub-flavored markdown.

Author: PlagueHO