Only add discourse link when configured

[jarrodmillman]

See scientific-python/specs#435 which is erroring with

2:31:02 PM:   ✖ FAIL fragment-check public/news/index.html --> #discourse-comments
2:31:02 PM:   | operator: fragment-check
2:31:02 PM:   | expected: id="discourse-comments"
2:31:02 PM:   |       at: public/news/index.html:134:79 <a href="#discourse-comments">...</a>

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[jarrodmillman]

@agriyakhetarpal I am not sure this is correct. Could you take a look?