Re-levelization: move non-docs content, rename content→docs

For better URLs, the content folder has been renamed 'docs' and all
other files have been moved up a level. Also, non-docs images have been
moved to the static folder at the top level where they belong.

Many relative paths had to be fixed to make this work.

docs/concepts/accounts/accounts.md

@@ -66,4 +66,4 @@ The typical way to get an account in the XRP Ledger is as follows:
     - [Manage Account Settings (Category)](../../tutorials/tasks/manage-account-settings/index.md)
     - [Monitor Incoming Payments with WebSocket](../../tutorials/get-started/monitor-incoming-payments-with-websocket.md)
 
-{% raw-partial file="/_snippets/common-links.md" /%}
+{% raw-partial file="/docs/_snippets/common-links.md" /%}