Document C++ code style and naming conventions

[Xrayez]

Your Godot version: Any.

Issue description: There's little information regarding existing code style used throughout Godot's C++ codebase in the docs, unlike GDScript and C#. This is important to document to ensure Godot follows consistent code style, and especially important for new contributors.

See godotengine/godot#33027 for consensus regarding C++ code style in Godot. To summarize:

Formatting

• No empty line after opening brace (enforced by clang-format).
• One line between function implementations (no tools found yet).
• Use braces on single-statement if / else blocks (might be doable with clang-tidy).
• No short case labels on a single line (can be enforced by clang-format).
• Switch/case use of brackets (undecided).

Code style

• snake_case for identifiers (variables).
• PascalCase for classes.
• SHOUTING_SNAKE_CASE for constants.
• Use p_ for method parameters, and r_ for parameters which are intended to "return" something (modified by the method).
• Naming of boolean setters/getters (no consensus yet, see godotengine/godot#33026).

URL to the documentation page (if already existing): Can be added to the "Code style guidelines" page.

[romlok]

It should also be noted that m_ is seemingly the preferred prefix for macro parameters. p_ and r_ are mentioned in the docs (though not in the coding style doc), but searching for m_ comes up blank.