[CloudSecurity][Docs][Serverless] Remove Defend for Containers (D4C) docs

[nick-alayil]

Summary

Remove all Defend for Containers (D4C) related documentation from Elastic Security Serverless documentation as part of the ongoing D4C deprecation process in Serverless.

Context

• D4C integration is being deprecated in Serverless
• This deprecation does not affect ECH documentation at this time
• ECH related changes will be addressed separately in 9.0.0 documentation

Required Changes

1. Remove the following documentation pages & sub-pages entirely:
   https://www.elastic.co/guide/en/serverless/current/security-d4c-overview.html

2. Update the billing documentation page:
   https://www.elastic.co/guide/en/serverless/current/security-billing.html#security-billing-cloud-protection

Remove the following references

• For [D4C](https://www.elastic.co/guide/en/serverless/current/security-d4c-overview.html), billing is based on how many agents (agent.id s) you use.
• and configuration drift prevention for Cloud Workloads.

Additional Requirements

Conduct a comprehensive search for "Defend for Containers" and "D4C" keywords throughout the Serverless documentation to identify any additional references that need removal