feature: add pydoctor to generate automatic documentation

[AlberTajuelo]

Added the following:

• pydoctor to create automatic documentation
• removed currencies list and added a link to currencies docs.
• improved CONTRIBUTING.md file
• removed duplicated license in README.md file

You will need to configure your GitHub page at Settings > Pages (https://github.com/bukson/steampy/settings/pages):

Select "Build and deployment" > "Source", the "Deploy from a branch" value.

In "Branch" select "gh-pages", select "/ (root)" and press the "Save" button.

Now, it will run the GitHub page workflow in the "gh-pages" branch and when it finishes, you just need to go to: https://bukson.github.io/steampy/

If you follow the previous steps, you can see how it looks on my GitHub page (https://albertajuelo.github.io/steampy/index.html).

Future steps to improve the documentation in this project:

• Remove inside the README file all functions and examples and place inside code.
• Add more documentation to each module/class/function.