docs: add documentation for SSE middleware

[wuhan005]

What needs to be improved? Please describe

• Add both English and Chinese versions of SSE middleware.

Update links to the docs on:

• The "Middleware" section on the main repository's README
• The "Getting help" section on each middleware repository's README
• flamego.dev/middleware

Why do you think it is important?

This is a new middleware for Flamego.

It was implemented in "dependency injection style" which is much different from the other Go web frameworks.

So it's necessary for us to provide documentation for developers.

Code of Conduct

• I agree to follow this project's Code of Conduct