Change write-up link for Contraception module to article

[matt-graham]

Fixes #1247

Changes link on documentation site write-ups page for Contraception module to link to published article in Studies in Family Planning.

Currently this removes the link to the previous .docx write-up but doesn't remove the actual file. Alternatives would be to keep the link to the .docx file in addition to the article link or to remove the .docx file entirely.

[tbhallett]

Thanks @matt-graham and @timcolbourn

I would say we should keep the link the original write-up docx file, and add a link to the journal article.

(Unless it's the case that the original write-up is now out-dated and/or contradicts the paper, ... @timcolbourn ..?).

[tbhallett]

Add a commit to that effect.