docs: migrate wiki into main repo

docs/Styling guide.md

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+Ironbar ships with no styles by default, so will fall back to the default GTK styles.
+
+To style the bar, create a file at `~/.config/ironbar/style.css`.
+
+Style changes are hot-loaded so there is no need to reload the bar.
+
+A reminder: since the bar is GTK-based, it uses GTK's implementation of CSS,
+which only includes a subset of the full web spec (plus a few non-standard properties).
+
+The below table describes the selectors provided by the bar itself.
+Information on styling individual modules can be found on their pages in the sidebar.
+
+| Selector       | Description                               |
+|----------------|-------------------------------------------|
+| `.background`  | Top-level window                          |
+| `#bar`         | Bar root box                              |
+| `#bar #start`  | Bar left or top modules container box     |
+| `#bar #center` | Bar center modules container box          |
+| `#bar #end`    | Bar right or bottom modules container box |
+| `.container`   | All of the above                          |