Document secret handling in Dioxus fullstack

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 `cfg`ed 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

Start in docs-src/0.4/en/reference/fullstack and review the existing fullstack documentation to choose whether this belongs in a new chapter or an anti-patterns section. Document how to keep server-only secrets such as API keys out of the client, including the stated cfg-based server separation; done means the guidance is present in the fullstack docs.

Written by the indexing model from the issue text.