Remove popular recommender documentation

[Copilot]

The popular recommender was removed in gorse-io/gorse#1080. This PR removes all related documentation.

Changes

Configuration

• Removed recommend.popular section and popular_window parameter
• Removed enable_popular_recommend from offline recommendation options
• Updated explore_recommend default from { popular = 0.0, latest = 0.0 } to { latest = 0.0 }

API Documentation

• Removed /api/popular and /api/popular/{category} endpoint documentation
• Removed popular endpoints from API reference tables

Concepts

• Removed "Popular Items" algorithm section
• Updated workflow diagrams to remove popular item flows
• Updated non-personalized recommender introduction from "two recommenders" to "one recommender"
• Updated cache size comments from "recommended/popular/latest" to "recommended/latest"

All changes applied to both English (src/docs/master/) and Chinese (src/zh/docs/master/) documentation.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove popular recommender documents</issue_title>
<issue_description>Remove popular recommender documents since it has been removed in gorse-io/gorse#1080</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Remove popular recommender documents #125

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