Add documentation for using HttpClient to send POST/PUT requests

[Whipstickgostop]

Either a new examples entry, or a new area in the C# page.
Or event add some way to link examples from pages, in order to link it directly from the Fetch URL sub-action page. 🤔

Will think this over...

Additional context
Rondhi's HttpClient docs

PUT Example:

// Create an object to send as JSON
var data = new
{
   Id = 1,
   Name = "John Doe",
   Email = "[email protected]"
};

// Serialize the object to JSON
var json = JsonConvert.SerializeObject(data);
var content = new StringContent(json, Encoding.UTF8, "application/json");

// Send PUT request
HttpResponseMessage response = await _httpClient.PutAsync("https://some-url.com", content);