[DOCS IA] Create a guide to the relationships between Defend, Agent, and Endpoint

[benironside]

What can we change to make the docs better?

There's a lack of clarity around the differences between the different components of Elastic Security's host protection software. We should create a guide that explains these relationships, and include it in the "Configure endpoint protection with Elastic Defend" section.

Let's consider two options to start: adding the information to this landing page, OR creating a new subpage within the section.

Assuming we go with the landing page option, let's expand it beyond just adding the security/defend/endpoint content, and provide more general description of this feature and how to find what they're looking for.

Another possible side-task: look for more opportunities to explain these components more clearly and deliberately whenever we mention them in other pages. Following up on work done in elastic/security-docs#5707 (in particular elastic/security-docs@14a5178).

Doc URL

Please include the doc URL and any other related information where applicable:
Doc URL: https://www.elastic.co/guide/en/security/current/endpoint-protection-intro.html

Which documentation set needs improvement?

ESS only

Software version