Fix outdated docs and add "URL encoding chapter"

[JOJ0]

This fixes links to code snippets in CONTRIBUTING.md chapter "Implementation Examples" and adds a chapter about the newly introduced way of formatting strings in the query() method of ApiRequest.

[JOJ0]

As usual, it's longer than I thought it will be. Give me your critics! ;-) Best viewed in its rendered form in the dev branch: https://github.com/JOJ0/synadm/blob/dev/CONTRIBUTING.md#command-design

I tried to follow the wording from the Python docs: https://docs.python.org/3.9/library/string.html#formatstrings

[JOJ0]

nitpicker noises

Thanks a lot! Nitpicking appreciated :-)

[JOJ0]

Unfortunately I forgot to finish this one. There might have been some things that still could be improved but due to lack of time I'm deciding now to better merge it than have inaccurate docs any time longer. I read through it and think it should be understandable by our contributors.