Handle relative links to markdown docs in source code

[jason0x43]

From theintern/intern#914:

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https://github.com/theintern/intern/blob/e7d20e54fd41aae03b159c5c911b77025cc80aad/src/lib/common/config.ts#L206

This link appears in the doc page

https://theintern.io/docs.html#Intern/4/api/lib%2Fcommon%2Fconfig/functionalcoverage

as

https://theintern.io/docs.html#Intern/4/api/lib%2Fcommon%2Fwriting_tests.md

which does not exist.

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The site should be able to handle doc links. It should probably assume .md files are of type 'doc' and are in docs/.