Document secret handling in Dioxus fullstack

[ealmloff]

We should document how to handle secrets that you want on the server but not on the client. Like API keys. It doesn't have to be a large section, but something about making sure that secrets are cfged out on the server would be useful.

We should add it to the fullstack docs here: https://github.com/DioxusLabs/docsite/tree/master/docs-src/0.4/en/reference/fullstack. It could be in a complete separate chapter or be part of a larger fullstack anti-patterns chapter