Include code files

#1524

It would be great to have a way to include code files into README.rst. By separating the code to another file, we can do static type checking etc. on our examples to ensure they are always running correctly.

(I don't what the point of adding a stale bot is, if nobody triages the issues here...)

Start by reading the referenced issue #1524 and the README.rst examples to determine the intended code-file inclusion behavior. Trace how README.rst is currently processed, then define and test a complete inclusion path whose behavior is clear for separated, statically checked examples.

索引モデルが issue の本文から書いたものです。