doc: Began documenting the code style

docs/CODE_STYLE.md

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+# Naming
+
+## Functions
+
+Functions (and methods) use `PascalCase`.
+Functions (but not methods) exposed in headers additionally use the prefix  `rt`: `rtLoadResource`.
+
+## Types
+
+Types (including C++ clases) use `snake_case`.
+Types exposed in header files use the prefix `rt_`: `rt_aio_handle`.
+This is often also done for "private" types in C/C++ files, but there it is not mandatory.
+
+## Variables
+
+Variables use `snake_case`.
+Global variables use the prefix `g_`, while file-scope global variables use a single underscore as their prefix `_`.
+Member variables of C++ classes (but not plain-old data C structs) use the prefix `m_`.
+
+## Macros
+
+Macros use `ALL_UPPER_CASE` with the `RT_` prefix.
+This is also used for enum options.
+
+## API and Interface Structs
+
+API structs (like `rt_render_backend_api`) are collections of function pointers,
+without any "object pointer" containing wrapped state.
+They use the suffix `_api`.
+
+Interface structs (like `rt_render_device_i`) also contain function pointers,
+and additionally an "object pointer", usually called `o`, that points to some
+stateful object. The function pointers in the interface usually take this object as their first parameter.