rewrite links: .md -> .html

[robrich]

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Short summary of your issue

Rewrite content to replace .md with .html during rendering

Input data for reproducing

Is it possible to get esdoc to rewrite relative url .md links to .html during rendering? The .md files live in GitHub which is indexed by Google. Building the docs creates the site which is also indexed by Google. If a user lands on the site, the .html links work. If the user lands on the GitHub repo, the .html links don't work because the actual relative file is a .md file.