Relates to #8383
* Convert the image tag topic to Markdown
* Convert the permissions usage to Markdown
* Convert the topic of snippets to Markdown
* Convert the streamfield topic to markdown
* Convert the writing templates doc to Markdown
This prevents duplicated headings in places where the second-level index page has intro blurbs for each subsection (and also means we're not listing out long multi-section pages in full in the index, but given how big these indexes are that's probably not a bad thing).
