add docs about serialization modes

[atilafassina]

this adds some docs around the new serialization modes introduced in 1.3.0 and 2.0.0-alpha.2

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Pull request overview

Adds documentation for SolidStart’s server function serialization modes and how they interact with CSP, linking the new configuration option from relevant server-function/security docs.

Changes:

• Add a new serialization section to defineConfig docs, describing json vs js modes and defaults.
• Cross-link serialization configuration from the "use server" reference and the Security guide.
• Minor formatting/whitespace adjustments in the touched MDX files.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
src/routes/solid-start/reference/server/use-server.mdx	Adds a link from server-function serialization docs to defineConfig#serialization and small formatting tweaks.
src/routes/solid-start/reference/config/define-config.mdx	Documents the new serialization config option (modes/defaults) and adds it to the parameters table.
src/routes/solid-start/guides/security.mdx	Adds CSP guidance referencing JSON serialization to avoid unsafe-eval, plus minor formatting fixes.

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[lxsmnsyc]

We can probably mention the data types we specifically support since seroval's compat table only shows what it can provide, but not by default.

https://github.com/solidjs/solid-start/blob/926336caaeddd8a8a72b73d3e524f8d9d3d414d2/packages/start/src/server/serialization.ts#L25-L37