Add attribute documentation for randomize_layout and no_randomize_layout

[Nixoncole]

First draft of the documentation for our Compiler Attributes.

Some notes:

• We found the best way to identify our proper "DocumentationCategory" by searching through the available options, and looking for ones that directly targeted structs, classes, and unions. This (https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Basic/AttrDocs.td#L2573) led us to the conclusion that we should take the category of DocCatVariable.
• Our next step is to go through their formal testing process for adding documentation of this type, but first wanted to know, what other details we should include in our description here? What things could we explain better?