Update ADK doc according to issue #866 - 2 - runner.run_debug()

[adk-bot]

This pull request adds a new section to the quickstart guide that explains how to use the run_debug() helper method for simplified agent debugging. This addresses item #2 in issue #866.

[koverholt]

My feedback on this PR:

• The content here (minus the "before run_debug" code sample) is a good start for doccing this feature

• We probably don't want to include this info in the (old) quickstart, but rather a new page such as "Run agents in code" or "Querying agents" under the https://google.github.io/adk-docs/runtime/ category

• No need to show the "before run_debug" code sample

• For cases that go beyond the functionality of run_debug, we should have a more comprehensive, canonical runner code sample on https://google.github.io/adk-docs/runtime/

• There is no standalone canonical runner code sample in https://github.com/google/adk-python/tree/main/contributing/samples, only code that's used within existing samples such as https://github.com/google/adk-python/blob/main/contributing/samples/hello_world/main.py#L33-L71