Add themes documentation

- Renamed all index.md to readme.md to make consuming the documentation
from the Github repo easier. VuePress treats index.md and readme.md the
same way when building the static pages.
- Added theme section overview documentation
- Added how to customize documentation
- Added custom style imports to support OpenBMC Web UI component
documentation to remove Bootstrap framework base styles that conflict
with the Vuepress theme
- Found occurrences of the word Sass in upper-case and changed to use
sentence case.

Signed-off-by: Derick Montague <derick.montague@ibm.com>
Change-Id: If5588f88a3d75761cc47e85a3fb0a582b31d373c
diff --git a/docs/.vuepress/config.js b/docs/.vuepress/config.js
index 8f94ff5..dd112e6 100644
--- a/docs/.vuepress/config.js
+++ b/docs/.vuepress/config.js
@@ -51,7 +51,7 @@
           ]
           }
         ],
-        "/themes/": [""]
+        "/themes/": ["", "customize"]
       },
     }
   };
\ No newline at end of file