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/request-formatting.md

@@ -137,4 +137,4 @@ The following types of changes are **non-breaking changes** and may occur withou
 - Adding a new field to a request or response, not including positional parameters.
 - Adding a new API method.
 
-{% raw-partial file="/_snippets/common-links.md" /%}
+{% raw-partial file="/docs/_snippets/common-links.md" /%}