docs: add stdout/stderr and index support to recipe directive

[lidavidm]

• Allow marking up stderr/stdout to be rendered in recipes
  • Syntax follows Go's (// Output:)
• Allow adding keywords in recipes that will get added into the Sphinx index
• Add script that parses stderr/stdout and places them in files next to recipes for test harnesses to use
• Enforce up-to-date output in CI
• Update Python test harness to validate stdout
• Update C++ test harness to validate stdout

Fixes #2082.

[kou]

+1