The contributing notes for the nu website is missing a style guide

[savente93]

It would also be nice to add some style guides for contributing to the book. Currently the style of e.g. code examples is a bit inconsistent. For example:

1. Code examples should use the nu formatting as in ```nu , currently this is not very consistent as I see none, bash, sh, nu and nushell used. I think it would be nice to have these consisten.
2. Another that might be nice is to use a consistent style for output formating of tables. E.g. "examples of table output should use rounded corners" (don't actually mean it needs to be rounded, just that we would use a consistent styling"