Creates static route for internalPath

[CarloDG80]

This PR makes easier to add route to static files and static behaviors.

    auto app = crow::SimpleApp();
    
    // Add static route by function
    app.static_file("/home", "home.html");
    // or add static route by MACRO
    CROW_STATIC_FILE(app, "/favicon.ico", "favicon.png");

[gittiver]

misses tests and documentation for new Macro

[CarloDG80]

Hi, how you suggest:

• I change TEST_CASE("send_file") to use CROW_STATIC_FILE
• I update docs/guides/static.md