docs: Move docs into docs/

docs/highlighters/root.md

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+zsh-syntax-highlighting / highlighters / root
+---------------------------------------------
+
+This is the `root` highlighter, that highlights the whole line if the current
+user is root.
+
+
+### How to tweak it
+
+This highlighter defines the following styles:
+
+* `root` - the style for the whole line if the current user is root.
+
+To override one of those styles, change its entry in `ZSH_HIGHLIGHT_STYLES`,
+for example in `~/.zshrc`:
+
+    ZSH_HIGHLIGHT_STYLES[root]='bg=red'
+
+The syntax for declaring styles is documented in [the `zshzle(1)` manual
+page](http://zsh.sourceforge.net/Doc/Release/Zsh-Line-Editor.html#SEC135).