- fix some broken refs that were lost in the migration to markdown files
- use lower_snake_case for all internal refs (most follow this convention)
- release process sub-section in docs should be named release schedule instead
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