Update docs for removed /preview_markdown route
What does this MR do and why?
Small change to update references to the removed
/groups/<group>/preview_markdown
/<group>/<project>/preview_markdown
in favor of
/groups/<group>/-/preview_markdown
/<group>/<project>/-/preview_markdown
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Related to #461439 (closed)