Add documentation for public non-method functions

- Rename `docs` folder to `doc` to match LDoc's new default
- Move stylesheet to root folder

- Rename `createRef` to `createProxy`
-- Change `metas` table (metatable storage) to `proxies` (proxy table
storage)
-- Add __newindex to proxy metatable that throws an error
-- Add doc comment for Proxy type in init.lua (LDoc doesn't yet render
it in the HTML output because it has no methods)

- Only add headers to request table once instead of with every request
- Add option to disable profiling of requests
- Add method to reset cache
- Add argument checking functions assertString and assertNumber
- Slightly re-organise utility functions used by the decompress function