Improve documentation and official website

[chrislearn]

• Add more example code to the source code。
• Newbies to the cors function are prone to encounter problems when using it, and the documentation especially needs to be improved.
• The documentation of openapi is still very incomplete and needs to be improved.

[josejachuf]

Add more about handler error, please. Use the thiserror and others

[CharlieBytesX]

Hello! In which topics of the documentation you need contributions?

[chrislearn]

@CharlieBytesX
There are many areas for improvement

• The current documentation of many middlewares simply gives example codes, but does not explain in depth how to use them.
• The OpenAPI documentation is not perfect

In short, I hope that the documentation can be more friendly to novices and make it easier for everyone to learn and use. Any optimization and improvement of the documentation is welcome.

[CharlieBytesX]

Thanks, do you have a standard for pull requests? Would like to help next week, where is the docs web page repo? Are you talking of internal code documentation or the web page documentation?

[chrislearn]

https://github.com/salvo-rs/website

This is the website repo, you can give PR to this repo.