_content: fix markdown styling with dark theme under /src

[elasticspoon]

This adds a template for markdown files under the go.dev/src path

Font color is set by a css variable under an Article selector.
The previous implementation did apply the Article class to pages
under the /src path.

By creating a default template pages will respect theme font colors.

Fixes golang/go#64929

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Patch Set 2: Commit-Queue+1

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This PR is being closed because golang.org/cl/553957 has been merged.