update overview, doc link to official API, quick start

[codetricity]

1. update quickstart and see if it is up to date
2. extend overview and include information on enum as well as overall benefit of theta-client
3. add link to documentation for theta-client https://ricohapi.github.io/theta-client/index.html

[AirTechWick]

Finished 2. and 3. in my pull request