Add .md extension to internal page links in docs

[becky-gilbert]

This PR adds the .md file extension to internal page links in the documentation. This change fixes a number of INFO statements that warn about unrecognized relative links when you build the docs.

Before:

After:

It also fixes a file path error in the webgazer-calibrate docs page.

[changeset-bot]

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Latest commit: a27c94e

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