Add Semaphore example for limiting the number of outgoing requests be…

[czzrr]

Fixes #6412.

[maminrayej]

Some better spacing could make it more readable and make it more consistent with other examples.

• A new line before ## Limit the number of outgoing ...
• A new line after ## Limit the number of outgoing ...
• A new line before This example uses ...
• A new line before starting the code block
• A new line before #[tokio::main]

Other than these, looks good to me.

[czzrr]

Some better spacing could make it more readable and make it more consistent with other examples.

* A new line before `## Limit the number of outgoing ...`

* A new line after `## Limit the number of outgoing ...`

* A new line before `This example uses ...`

* A new line before starting the code block

* A new line before `#[tokio::main]`

Other than these, looks good to me.

I initially had this spacing, but rustfmt complained.
Turns out it was space after ///.
Fixed it.