docs: add docs for RateLimiter

[xialeistudio]

Add a RateLimiter quickstart to docs.
#87

[mustafasegf]

hey, while this code compiles, it gives 403 error while trying because the host header isn't modified to one.one.one.one. It would be better for the example to also add

    async fn upstream_request_filter(
        &self,
        _session: &mut Session,
        upstream_request: &mut RequestHeader,
        _ctx: &mut Self::CTX,
    ) -> Result<()> {
        upstream_request.insert_header("Host", "one.one.one.one").unwrap();
        Ok(())
    }

just like in the https://github.com/cloudflare/pingora/blob/36e09ca389dac053948722a8ed24caa011495439/docs/quick_start.md

[xialeistudio]

hey, while this code compiles, it gives 403 error while trying because the host header isn't modified to one.one.one.one. It would be better for the example to also add

    async fn upstream_request_filter(
        &self,
        _session: &mut Session,
        upstream_request: &mut RequestHeader,
        _ctx: &mut Self::CTX,
    ) -> Result<()> {
        upstream_request.insert_header("Host", "one.one.one.one").unwrap();
        Ok(())
    }

just like in the https://github.com/cloudflare/pingora/blob/36e09ca389dac053948722a8ed24caa011495439/docs/quick_start.md

Thanks to your valuable reminder, I have promptly addressed the issue.