Styleguide page for content formatting options

[thibaudcolas]

As a board member entering content in the site’s CMS, it’s really hard to know what different formatting options will look like once rendered on the page:

• Different HTML elements
• Different syntax of reStructuredText

Further, some of the content on the site doesn’t support previewing - so you either get it right the first time, or RSS feed subscribers can get broken content.

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I can’t manage Assignees on this repo but I’d like to address this myself by introducing new styleguide content, with a recap of all formatting options on the site.

• I’ll start with a flatpage so I can experiment easily.
• If it seems valuable enough, I’ll then move the content to the existing styleguide page (see also Style guide is outdated #1033).
• Also to investigate: how differently content renders across different parts of the site.