[docs] Update style.md to reflect current Skia practices

- Document full repo-root #include paths, include/private/base/, and sorting via tools/rewrite_includes.py.
- Add guidance on using git clang-format as a helpful guide while noting that manual formatting is fine when it improves readability.
- Expand modern Skia namespace documentation (skgpu::graphite, skgpu,skcpu, sktext, SkSL, skia_private).
- Note preference for scoped enums (enum class) and anonymous namespaces in implementation files.
- Note practical usage of single-line if guards and compact tabular
 switch cases.
- Update outdated code examples (remove blank line before public:, place && at end of wrapped line).

Change-Id: I15ac4552b2c51b6125f0791fb2134e000ea97b1d
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1380516
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>