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

PlagueHO wants to merge 2 commits intogithub:stagedgithub/awesome-copilot:stagedfrom

PlagueHO:add-copilot-instructions-markdown-gfmPlagueHO/awesome-copilot:add-copilot-instructions-markdown-gfmCopy head branch name to clipboard