[Internal]: Update Security Assistant Knowledge Base Docs to include added support of `text` fields for Index Entries

[spong]

Description

In the below PR's we've added support for selecting text fields (in addition to semantic_text when creating Index Entries in the Security Assistant Knowledge Base:

elastic/kibana#231376
elastic/kibana#231904

We currently have these docs around using Index Entries, so they'll need to be adjusted to include the support of this new field type. The images will also need to be updated as the input label has changed, and the suggestions now include the field type as well.

Resources

PR's:
elastic/kibana#231376
elastic/kibana#231904

Existing docs:
https://www.elastic.co/docs/solutions/security/ai/ai-assistant-knowledge-base#knowledge-base-add-knowledge-index

Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

Feature differences

No feature differences between deployment methods.

What release is this request related to?

9.1

Serverless release

Week of April 25

Collaboration model

The documentation team

Point of contact.

Main contact: @spong

Stakeholders: @elastic/security-generative-ai

[benironside]

Fixed in #2616