language.md: reorganize stdlib tables by class name instead of gem
name so users can quickly find "does mruby have Time/File/Set?"
capi.md: fix mrb_protect example (mrb->exc is cleared after protect,
so mrb_print_error does not work; show mrb_inspect instead); fix
fiber yield example to show correct usage as return value.
gc.md, compiler.md, vm.md: add "read this if" guidance paragraphs
to help developers decide whether they need each document.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto