[7.17] fix: replace ndjson.org with ndjson spec (backport #3997) (#3998)

* fix: replace ndjson.org with ndjson spec (#3997)

the original website expired and it's currently serving malicious content.

(cherry picked from commit 698d4e0)

# Conflicts:
#	docs/en/serverless/apm/apm-server-api/api-events.mdx

* Delete docs/en/serverless/apm/apm-server-api/api-events.mdx

---------

Co-authored-by: kruskall <[email protected]>
Co-authored-by: Brandon Morelli <[email protected]>

Author: 3 people

docs/en/apm-server/api-events.asciidoc

@@ -13,7 +13,7 @@ Events can be:
 * Metrics
 
 Each event is sent as its own line in the HTTP request body.
-This is known as http://ndjson.org[newline delimited JSON (NDJSON)].
+This is known as https://github.com/ndjson/ndjson-spec[newline delimited JSON (NDJSON)].
 
 With NDJSON, agents can open an HTTP POST request and use chunked encoding to stream events to the APM Server
 as soon as they are recorded in the agent.