[7.6] Roadmap for inference documentation

[szabosteve]

• Conceptual overview: [DOCS] Adds inference conceptual documentation #758
• Inference in API docs: [DOCS] Adds GET, GET stats and DELETE inference APIs elasticsearch#50224
• Inference in pipeline processors docs: [DOCS] Adds inference processor documentation elasticsearch#50204
• Related attributes [DOCS] Adds inference related attributes docs#1616

[szabosteve]

Inference settings: elastic/elasticsearch#49309