Add rbxgameasset to asset url types

[ccuser44]

Changes

Adds rbxgameasset to the documentation.

Posted from the old game asset content page.

Checks

By submitting your pull request for review, you agree to the following:

• This contribution was created in whole or in part by me, and I have the right to submit it under the terms of this repository's open source licenses.
• I understand and agree that this contribution and a record of it are public, maintained indefinitely, and may be redistributed under the terms of this repository's open source licenses.
• To the best of my knowledge, all proposed changes are accurate.

[IgnisRBX]

I made some minor edits, particularly that the "URL" needs to omit the file format (such as just /Potion and not /Potion.png). Thanks for the contribution!