docs(website): unify example code blocks to JEP-512 style

[bengbengbalabalabeng]

Purpose of the pull request

Unify the code style of all example code blocks in the documentation to follow a consistent JEP-512 (Compact Source Files and Instance Main Methods) pattern, replacing the previously varied @Test method styles.

What's changed?

• Before:

// Style 1
public static void main(String[] args) {
    // .....
}

// Style 2
@Test
void readAndWrite() {
    // ......
}

• After:

void main() {
    // ......
}

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[alaahong]

will the replacement making confuse with psvm ?

[bengbengbalabalabeng]

Thank you for the reminder! The purpose of switching to the JEP‑512 style this time is mainly to keep the example code in the documentation structurally consistent, and it is not intended to imply any connection between these examples and PSVM.

In the updated example, void main() is used only to improve readability; it does not represent the actual entry-point method, nor does it alter the semantics of the example. Therefore, it will not be confused with PSVM.

[alaahong]

Generally good, only on concern is the sample only support on JDK 25+, but current major user are from java 8 - 21...any confuse while following guide to code?

[delei]

Generally good, only on concern is the sample only support on JDK 25+, but current major user are from java 8 - 21...any confuse while following guide to code?

Let’s keep up with the times. LOL.

[bengbengbalabalabeng]

Or provide a supplementary note: indicate that the void main() pattern is used only to simplify and unify the example code style, and does not mean that this feature supports only Java 25+ versions.