docs: ensure "rafiki admin auth optional" is documented adequately

[melissahenderson]

The purpose of this issue is to ensure the documentation appropriately covers #2883 and the related comments made here: #2875 (comment) (summary of comments below).

Auth/Kratos is currently optional. In the local playground, Kratos is explicitly disabled, which is something we need to mention because it could be confusing to talk about registering users when most people won't encounter that step. For users who want to explore what auth is like locally, they'd run a separate command:
pnpm localenv:compose:adminauth up

Outside of the local playground, Auth/Kratos is enabled by default. This is something we should mention in the Rafiki Admin user guide. Something along the lines of "Here is the auth info. We advise you to use it, but if you want, you can disable it."

One or more pages in the user docs and READMEs may need updating.