Add initial docs

docs/development/creating_plugins.md

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+# Creating Plugins
+
+TODO

docs/development/creating_themes.md

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+# Creating Themes
+
+Themes need to define the following variables.
+
++ `WISH_DEFAULT_FG`
++ `WISH_DEFAULT_BG`
+
+Plugins will fallback to these variables if they are not themed.
+
+Themes also need to define variables for plugins. For each plugin named `abc`, the theme needs to define `WISH_ABC_BG` and `WISH_ABC_FG`. Themes can optionally iterate over `WISH_PLUGINS` array to ensure to theme all elements. Check existing [themes](https://gitlab.com/ceda_ei/wish/tree/master/themes).