docs(core): correct example code block formatting

[csvn]

The example for the CSP middleware looks a bit broken currently on JSR, due to the @example markdown is not wrapped in a code block.

Changes

• Improves formatting a bit, to be less compact
• Wraps the example in a code block ```ts
• Adds a title to the @example - "Basic usage" - To avoid the example being labeled as "Example1" on JSR

[csvn]

Hmm... giving titles to examples seems to break indentation of examples when using VSCode, which is kinda bad... microsoft/TypeScript#43955.

Which is more important, nicer documentation rendering on JSR, or correct rendering of tooltip in VSCode?

[marvinhagemeister]

LGTM, we should make our LSP better to handle example code blocks.