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/tutorials/get-started/monitor-incoming-payments-with-websocket.md

@@ -513,4 +513,4 @@ Many programming languages have libraries for sending and receiving data over a
     - [Transaction Metadata](../../references/protocol/transactions/metadata.md) - Summary of the metadata format and fields that appear in metadata
     - [Transaction Results](../../references/protocol/transactions/transaction-results/transaction-results.md) - Tables of all possible result codes for transactions.
 
-{% raw-partial file="/_snippets/common-links.md" /%}
+{% raw-partial file="/docs/_snippets/common-links.md" /%}