Document automatic formatting with clang-format in Shaders style guide

[Calinou]

• See Add .clang-format and use it to format shader files godot-demo-projects#1249
  and Add a Godot shader code formatter godot-vscode-plugin#843.

[mhilbrunner]

Thank you!

[mhilbrunner]

Cherrypicked to 4.5 in #11337 and to stable in #11338.