Add note about Security in SolidStart apps

[atilafassina]

This PR adds a few quick notes on security best practices for any kind of SolidStart app.

It's by no means a comprehensive guide an probably each section of this guide can (and should!) be expanded into its own entry and used to bulk up the APIs it mentions and require to work. It's also not an exhaustive guide either, plenty of practices have not been mentioned (yet!) and should be. This only the beginning to pave the way into adding better documentation and highlighting the DX around securing a SolidStart app.

As a follow-up to this PR, I'd expect these sections to be expanded into their own entries and this guide being expanded into its own section, as in:

security
├── csrf
├── csp
├── cors
├── xss
└── ...

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