Update Maintenance Mode code to 504

[morganestes]

When a site is in Maintenance Mode on the server side (either through actual maintenance or because it's been taken offline for various reasons), the server response is

HTTP/1.1 504 Target in maintenance.

This change updates that code and move the section down to the other 504 examples.

Summary

5xx Level Errors - Corrects the server response code for Maintenance Mode.

Effect

May break links since the anchor will change.

Release

• When ready

---

Post Launch

Do not remove - To be completed by the docs team upon merge:

• Redirect /old-path/ => /new-path/ (if applicable)
• Include/exclude pages ^ respectively within docs search service provider (if applicable)
• For Heroes - add a props post to the discussion board.
• Remove from the project board

[rachelwhitton]

Thank you!