Add instructions to what need to `volumes` in the documentation for containerized deployments

**Describe the enhancement:**

Add instructions to mount `/var/run/docker.sock` and more about `volumes` in the documentation for containerized deployments.

**Describe a specific use case for the enhancement or feature:**

- Assume that there is already an `ElatsicSearch` and `Kibana` instance locally, and `Elastic Agent` is being configured.
```yaml
version: "3.8"
services:
elasticsearch-node01:
image: elastic/elasticsearch:8.1.2
environment:
bootstrap.memory_lock: true
ELASTIC_PASSWORD: "1234AF57-35CA-B2E0-BE8B-12345D5C929E"
volumes:
- elasticsearch-data01:/usr/share/elasticsearch/data
- elasticsearch-config01:/usr/share/elasticsearch/config:rw
ports:
- "9200:9200"
- "9300:9300"
ulimits:
memlock:
soft: -1
hard: -1
kibana:
image: elastic/kibana:8.1.2
environment:
XPACK_SECURITY_ENCRYPTIONKEY: fc12345ea123eb1234dac7b12345ac12e
XPACK_ENCRYPTEDSAVEDOBJECTS_ENCRYPTIONKEY: 1bb12c4e9ed1c12ae4a8ead1234567f1
XPACK_REPORTING_ENCRYPTIONKEY: af5cbdd2e2f4da12a72f12f1e123456f
ports:
- "5601:5601"
volumes:
- kibana-data:/usr/share/kibana/data
- kibana-config:/usr/share/kibana/config
elastic-agent-node01:
image: elastic/elastic-agent:8.1.2
restart: always
user: root
environment:
FLEET_SERVER_ENABLE: true
FLEET_SERVER_ELASTICSEARCH_HOST: https://172.20.0.2:9200
FLEET_SERVER_SERVICE_TOKEN: "AAEAAWVsYXN0aWMvZmxlZXQtc2VydmVyL3Rva2VuLTE2NDkzMzUzMjQ5NzY6Q09FUU92aVpSRHl4QmVuNGRkQ2ctdw"
FLEET_SERVER_POLICY_ID: fleet-server-policy
FLEET_SERVER_ELASTICSEARCH_CA_TRUSTED_FINGERPRINT: "64f60363137d9620b7b864b145823e2ddb195634768c37886cfd26d6e10a1e7a"
FLEET_SERVER_INSECURE_HTTP: true
```
- When deploying Elastic Agent when configuring Fleet, there will be an unintuitive log in the docker log.

```shell
{"log.level":"warn","@timestamp":"2022-04-07T13:25:03.974Z","log.logger":"tls","log.origin":{"file.name":"tlscommon/tls_config.go","file.line":105},"message":"SSL/TLS verifications disabled.","ecs.version":"1.6.0"}

{"log.level":"info","@timestamp":"2022-04-07T13:25:04.224Z","log.logger":"composable.providers.docker","log.origin":{"file.name":"docker/docker.go","file.line":43},"message":"Docker provider skipped, unable to connect: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?","ecs.version":"1.6.0"}

{"log.level":"info","@timestamp":"2022-04-07T13:25:04.225Z","log.logger":"api","log.origin":{"file.name":"api/server.go","file.line":62},"message":"Starting stats endpoint","ecs.version":"1.6.0"}
```

- This is implying that the user mounts `/var/run/docker.sock:/var/run/docker.sock`, but there is no related introduction in https://www.elastic.co/guide/en/fleet/8.1/elastic-agent-container.html .

- At the same time, the documentation does not mention `volumes` mounts like `elastic-agent-log:/usr/share/elastic-agent/state/data/logs` to persist logs. `Elastic Agent` itself has the function of collecting system information, but the documentation does not indicate what directory should be mounted to help `Elastic Agent` collect information. I think this is an improvement point for the documentation .

Start with the Elastic Agent container deployment documentation at the linked elastic.co guide and compare it with the Docker Compose example and logs in this issue. Document the relevant /var/run/docker.sock and volume mounts, including persistence and host-collection guidance, so the containerized deployment instructions cover the reported setup.

Written by the indexing model from the issue text.