Most of these are no longer applicable due to the introduction of Wagtail guide, where possible the new guide links are used now
- Re-add missing docs image for extending Draftail
- There is no Guide page (yet) for Inserting videos or images so those links have been removed
- Fix some additional reference issues where the page name and reference were causing issues
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
