Follow-up to #8307. It appears that the original code was designed for a StreamField setup where the `rich_text` block is not a RichTextBlock, but a StructBlock with a RichTextBlock child also named `rich_text`. #8307 correctly removed this redundant wrapper from page_to_streamfield, but there are corresponding changes required for the rest of the helper methods.
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
