Enhance the docs

[lundberg]

• User Guide
  • Mock HTTPX
    • Add Using the pytest Fixture section
  • Routing Requests
    • Mention that the order of added routes matters as discussed in
      Mocking optional parameters is dependant on registration order #129
  • Mocking Responses
    • Mention identical API to the python built-in Mock
      e.g. side effect has precedence over return value, and iterable stop iteration vs return value.
    • Add Streaming Responses section with some examples, relates to
      Moved to Consider adding a MockStream #156
    • Add Mock using an app
    • Document new optional route argument in side effects