Document C++ code style and naming conventions

**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"](https://docs.godotengine.org/en/latest/community/contributing/code_style_guidelines.html) page.