docs: add v shader section, add help text (#12594)

cmd/v/help/shader.txt

@@ -0,0 +1,17 @@
+Usage:
+  v shader [options] [DIRECTORY / FILE]
+
+Examples:
+  v shader .
+  v shader examples/sokol/02_cubes_glsl/cube_glsl.glsl
+
+  Compile shaders in sokol's annotated GLSL format to C headers for use with sokol based apps.
+  Sokol based apps include all apps using V's gg module.
+
+Options:
+  -h, --help                     Show this help text.
+  -v, --verbose                  Be verbose about the tools progress.
+  -u, --force-update             Force update of the sokol-shdc tool.
+  -l, --slang <multiple strings> Shader dialects to generate code for. Default is all.
+                                 Available: 'glsl330', 'glsl100', 'glsl300es', 'hlsl4', 'hlsl5', 'metal_macos', 'metal_ios', 'metal_sim', 'wgpu'
+