Remove theme list from docs

docs/content/documentation/themes/themes-list.md

@@ -1,9 +0,0 @@
-+++
-title = "List of themes"
-weight = 40
-+++
-
-The following themes are available for Gutenberg:
-
-- [Hyde](https://github.com/Keats/gutenberg-hyde)
-- [Materialize](https://github.com/verpeteren/gutenberg-materialize)