DOC-4232 stream code examples

[andy-stark-redis]

Go versions of the examples in this page.

Note: it's difficult to make the output from the stream commands work well with Go testable examples because of the nondeterministic timestamps. I've tried to work around this by explicitly setting the entry IDs to match the ones from the CLI example where possible. However, for the examples that show XAdd without setting the IDs, I've just put the fmt.Println() statements inside comments to prevent the output being tested. Doing this also means that some return values from the examples are unused, so I've added an UNUSED() function to allow the examples to compile. If there's a better approach to this then please let me know :-)

Also, because I needed to set the IDs, I placed several examples together in the same function in a few places (ie, to avoid making the file huge with loads of XAdd calls at the start of each example function). This displays OK in the doc page, but let me know if it's a problem from a coding/testing point of view.