doc: Solidity class fields to Rust struct

[0xf333]

Fix solidity example and update ink example to conform with v4

Description

• Added SPDX license identifier for Solidity.
• Fixed the syntax for the if condition in Solidity.
• Fixed event emit syntax.
• Added no_main attribute in ink! contract.

[0xf333]

@SkymanOne

This should probably be fixed in a separated pull request since it's not part of this PR scope.

Check Markdown links / markdown-link-check (pull_request) 

Let me know what you think.

[0xf333]

@SkymanOne

This should probably be fixed in a separated pull request since it's not part of this PR scope.

Check Markdown links / markdown-link-check (pull_request) 

Let me know what you think.

markdown-link-check failure has been fixed in https://github.com/paritytech/ink-docs/pull/273 https://github.com/paritytech/ink-docs/pull/272 https://github.com/paritytech/ink-docs/pull/271

Reason for separated PR is to ensure that each link issue was resolved independently, providing a clearer history.