Support markdown for all docs and guides pages

[aliabd]

Have every docs / guide page on the website also accessible as markdown through the same url appended with .md

This is useful for:

• creating an llms.txt map of the docs that llms can crawl
• adding copy page / open in huggingchat buttons with simpler logic that just copy the markdown content
• allowing claude code and similar tools to fetch more token efficient docs