Add a Code Block

[dacharyc]

Cover how to create a code block.

The bulk of this is slide 29 in this presentation: https://docs.google.com/presentation/d/1cVA1jDNlLywiEGxPhimDDmzm20yhT0zTAlTKK2TBKV8/edit?usp=sharing

However, we should also link to sections on Adding Tabs for different languages and Re-Using Content with Includes (slides 27 and 30)

[biniona-mongodb]

https://github.com/mongodb/docs-meta/blob/markup-guide/source/markup/Code%20Blocks.txt

[biniona-mongodb]

I can claim this one