Add texdoc

`texdoc --print-completion zsh` will generate something like

```
compdef __texdoc texdoc

__texdoc() {
  ...
}
```

So put

```
(( $+functions[__texdoc] )) || eval "$(texdoc --print-completion zsh)" && __texdoc
```

can make it out of box for users to avoid add `eval "$(texdoc --print-completion zsh)"` to their zshrc

src/_texdoc

@@ -0,0 +1,2 @@
+#compdef texdoc
+(( $+functions[__texdoc] )) || eval "$(texdoc --print-completion zsh)" && __texdoc