Update runtime doc file

This updates and reformats the doc file for the runtime package. The
list of things that need to be implemented for a compiler runtime to
work has been updated in accordance with recent changes to runtime
(notably `cstring16` and `string16` types and `#load_directory`). Some
copy-paste errors around string-comparison procedures have been fixed.

The file has been renamed to `doc.odin` as per convention, the comment
has been moved to the top of the package declaration and the comment is
now markdown-formatted.

Also wrote down a little map to maybe slightly help with understanding
navigation within package runtime's files and added a few more
explanations as to why that list of declarations exists and how to use
it.

The list of declarations has been detailed to some degree, explaining
under what conditions the declarations are required.

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