Consistent styling across README

[mikomikotaishi]

This PR makes styling in the README files more consistent:

• Add missing whitespace, such as between function parameters, between assignment operators, and between comment lines
• Remove the drogon:: prefix (when it is already imported by using namespace drogon;)
• Capitalise HTTP, HTTPS, JSON, HTML, WebSocket
• Label classes in monospace (such as HttpController, HttpSimpleController, etc.)
• Clearly indicate HttpResponse::newHttpJsonResponse() as returning HttpResponsePtr
• Use concise namespace declarations (namespace api { namespace v1 { /* ... */ } } -> namespace api::v1 { /* ... */ }), as the library is minimum C++17