Convert docdock theme from submodule to native files and fix Hugo compatibility

themes/docdock/exampleSite/content/content-organisation/customize-style/themestyle.md

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++++
+title = "Theme styles"
+description = "change theme style"
++++
+
+In site configuration file, you can set a subtheme name of this theme to load a specific css.
+
+add a param `themeStyle = "STYLE_NAME"` in the `[params]` part of config.toml file.
+
+
+## Style "original"
+```
+[params]
+themeStyle = "original"
+```
+
+![green docdock](/variant-gray.png)
+
+## Style "flex" (work in progress)
+```
+[params]
+themeStyle = "flex"
+```
+
+![flex](/style-flex.png)