Task(5048): Add documentation for Custom Endpoint Providers configuration

[udhayarekha01]

Description

This PR adds comprehensive documentation for configuring and using custom endpoint models in Code Studio, enabling users to connect to external model providers using their own API keys.

Changes

Documentation

• New Guide: Created custom-endpoint-providers.md
  • Overview of custom endpoint provider functionality
  • Use cases and when to use custom endpoints
  • Prerequisites for setup
  • Step-by-step configuration walkthrough (5 steps)
  • Visual guidance with screenshots

Navigation

• Updated code-studio-ide-toc.html to include link to the new custom endpoint providers guide

Assets

• Added 5 supporting images:
  • custom-endpoint-setting.png - Managing language model interface
  • custom-endpoint-selection.png - Selecting custom endpoints from dropdown
  • custom-endpoint-api.png - Configuring API key and endpoint type
  • custom-endpoint-config.png - JSON configuration example
  • custom-endpoint-model.png - Model selection in chat panel

Motivation

Provides clear guidance for users who need to integrate models from external providers (e.g., Claude, custom APIs) by supporting Chat Completions, Responses, and Messages endpoint types.

Benefits

• Helps users configure custom models with their own API keys
• Supports multiple endpoint types for different providers
• Includes visual step-by-step instructions for ease of use

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