[8.x](backport #4246) [Observability docs] Change 'beats' namespace to 'elastic-agent' (#4256)

* [Obs docs] Change 'beats' namespace to 'elastic-agent' (#4246)

(cherry picked from commit 35cba81)

* Delete docs/en/serverless directory

---------

Co-authored-by: David Kilfoyle <[email protected]>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: 3 people

docs/en/integrations/developer-workflow-fleet.asciidoc

@@ -101,7 +101,7 @@ To run the Fleet Server Docker container:
 +
 [source,terminal]
 ----
-docker run -e KIBANA_HOST=http://{YOUR-IP}:5601/{BASE-PATH} -e KIBANA_USERNAME=elastic -e KIBANA_PASSWORD=changeme -e ELASTICSEARCH_HOST=http://{YOUR-IP}:9200 -e ELASTICSEARCH_USERNAME=elastic -e ELASTICSEARCH_PASSWORD=changeme -e KIBANA_FLEET_SETUP=1 -e FLEET_SERVER_ENABLE=1 -e FLEET_SERVER_INSECURE_HTTP=1 -p 8220:8220 docker.elastic.co/beats/elastic-agent:{VERSION}
+docker run -e KIBANA_HOST=http://{YOUR-IP}:5601/{BASE-PATH} -e KIBANA_USERNAME=elastic -e KIBANA_PASSWORD=changeme -e ELASTICSEARCH_HOST=http://{YOUR-IP}:9200 -e ELASTICSEARCH_USERNAME=elastic -e ELASTICSEARCH_PASSWORD=changeme -e KIBANA_FLEET_SETUP=1 -e FLEET_SERVER_ENABLE=1 -e FLEET_SERVER_INSECURE_HTTP=1 -p 8220:8220 docker.elastic.co/elastic-agent/elastic-agent:{VERSION}
 ----
 +
 Ensure you provide the `-p 8220:8220` port mapping to map the Fleet Server container's port `8220` to your local machine's port `8220` in order for Fleet to communicate with Fleet Server.

docs/en/observability/synthetics-private-location.asciidoc

@@ -87,7 +87,7 @@ To pull the Docker image run:
 
 [source,sh,subs="attributes"]
 ----
-docker pull docker.elastic.co/beats/elastic-agent-complete:{version}
+docker pull docker.elastic.co/elastic-agent/elastic-agent-complete:{version}
 ----
 
 endif::[]
@@ -116,7 +116,7 @@ docker run \
   --env FLEET_ENROLLMENT_TOKEN={enrollment-token} \
   --cap-add=NET_RAW \
   --cap-add=SETUID \
-  --rm docker.elastic.co/beats/elastic-agent-complete:{version}
+  --rm docker.elastic.co/elastic-agent/elastic-agent-complete:{version}
 ----
 
 endif::[]