FastAPI and testing example with supadantic (docs)

[AlexPozh]

closes #40

[makridenko]

Can we make endpoints more RESTful?

• GET /authors/ (Authors list)
• GET /authors/:id/ (Retrieve author)
• POST /authors/ (Create author)
• PATCH /authors/:id/ (Update author)
• DELETE /authors/:id/ (Delete author and books)
• GET /authors/:id/books/ (Author's book list)
• GET /authors/:id/books/:book_id/ (Retrieve book)
• POST /authors/:id/books/ (Create book)
• PATCH /authors/:id/books/:book_id/ (Update book)
• DELETE /authors/:id/books/:book_id/ (Delete book)

Also, in books endpoints we can catch two different exceptions, Author.DoesNotExist and Book.DoesNotExist. It would be nice if we will show it. Two different responses - Author not found or book not found