Move some files, add small theme convert document.

doc/theme3/convert.md

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+# Convert old themes to the new format
+
+**Rofi** 1.4 can still read in and convert the old theme format. To read the old format, convert it, and dump it into a
+new file run:
+
+```bash
+rofi -config {old theme} -dump-theme > new_theme.rasi
+```
+
+This gives a very basic theme, with the colors from the old theme.
+You can preview the theme using:
+
+```bash
+rofi -theme new_theme.rasi -show run
+```
+
+

doc/theme3/index.md

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+# Rofi Advanced Style Information
+
+**rasi** is **rofi**'s new theme format (theme format version 3).
+
+* [Specification](specification.md)
+* [Tutorial](tutorial.md)
+* [Covert old themes](convert.md)