[Documentation] Document the official MIME type for `.gjs` files

I’m working on improving MIME type support for modern web framework source files across tooling, servers, and content-type databases.

Currently, there does not appear to be an officially documented MIME type for `.gjs` files.
Some tools/frameworks use conventions such as `text/x-`, but there is no authoritative reference from the Ember project itself.
This create ambiguity for other projects to rely on a consistent mapping.

Having an official statement would help:
- MIME databases include the mapping
- web servers and CDNs configure correct content types
- editors and language servers align
- reduce ecosystem fragmentation

Would the Ember project be open to document an official recommendation for the MIME type of `.gjs` files?

This does not necessarily require [IANA](https://www.iana.org) registration though.
A short note in the docs would already provide a reliable reference for downstream projects I think.

If another MIME type from `text/x-ember` (like `text/plain`, `text/html`, `text/html+ember`, ...) is preferred, documenting that choice would be equally helpful.

Thank you for your work and for considering this!