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/references/http-websocket-apis/api-conventions/error-formatting.md

@@ -123,4 +123,4 @@ All methods can potentially return any of the following values for the `error` c
 - `unknownCmd` - The request does not contain a [command](../index.md) that the `rippled` server recognizes.
 - `wsTextRequired` - (WebSocket only) The request's [opcode](https://tools.ietf.org/html/rfc6455#section-5.2) is not text. <!-- SPELLING_IGNORE: opcode -->
 
-{% raw-partial file="/_snippets/common-links.md" /%}
+{% raw-partial file="/docs/_snippets/common-links.md" /%}