Create a Drogon Blog Example (Frontend)

[rbugajewski]

1. Create an example project which resembles a blog as an introduction for new Drogon users.
2. Extend the documentation to contain a small, comprehensive blog example tutorial.

[rbugajewski]

Authentication in some form should be part of the example project as we currently have many questions regarding this topic.

[marty1885]

Just for documentation purpose. The login_session example shows how one can do authentication in a classic form/server side rendering style. But the code is not production ready (ex. Using MD5 for password hashing, no DB access, etc)

https://github.com/an-tao/drogon/blob/b654e35e51907281d6b24d214577f759ff05e96c/examples/login_session/main.cc#L33-L59