Improve PyOTA documentation

[lzpap]

After the first iteration of docs update #260 that dealt with API commands, continue the process of making the documentation more readable for users and improve it in general.

Specific areas to improve:

• Add high-level explanation of IOTA concepts that link to the official docs site.
• Explain the PyOTA data types and their implementation details. It should be clear what is used for what, and how. For example, document TryteString class and its sub-classes and include examples how to use them.
• Improve documentation on how address generation works in PyOTA, with actual scenarios that show the corner cases, after snapshot problem, etc.
• Explain adapters, how are they used and what happens upon API command execution.
• Document filters.
• Add tutorials, examples at the end about frequently used operations or popular use case scenarios. Maybe include the python community workshop with some extended examples?
• Make the documentation more visually appealing by including pictures. For example, to demonstrate the process of sending a transaction from scratch like this pic does, or to show what options a user has to construct a bundle like this.

Ideas and feedback on how to improve the docs are more than welcome! 👋