[agent builder] Update models docs for 9.4 model management UI

[charlotte-hoblik]

Updates models.md to match the new model management experience in Kibana 9.4. Users now manage models from Stack Management → Model management, which contains the Elastic inference and External inference screens. This PR:

• Surfaces inference endpoints as the recommended way to add a model in 9.4 and later, with clear instructions for both Elastic-managed models (Elastic inference) and third-party providers (External inference).
• Keeps Generative AI connectors documented as a still-supported, universal fallback that works on every deployment and version.
• Drops "connector"-only framing from sections where the new UI talks in terms of models and endpoints.

Closes #6295

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🔍 Preview links for changed docs

• explore-analyze/ai-features/agent-builder/glossary.md
• explore-analyze/ai-features/agent-builder/models.md

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[charlotte-hoblik]

Could I get your eyes on this when you have a moment?

@pgayvallet — small bit of context in case it's been a while: you suggested adding an Inference endpoint glossary entry in #6175. We agreed to wait until models.md was rewritten for the 9.4 model management UI. That's done here, and the glossary entry is included.

@leemthompo — pulling you in for the Agent Builder 9.4 angle.

@lcawl — would love a sanity check on the inference-endpoint vs connector split in models.md.

Thank you!

[pgayvallet]

LGTM