docs: add scheduling framework developer guide with example plugins

[gyliu513]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Summary

Adds a developer guide and working examples that demonstrate how to extend the
scheduling framework with custom out-of-tree plugins and how to build a custom EPP
binary.

What's included

• examples/scheduler/ — A standalone demo (make run-example EXAMPLE=scheduler)
  that wires together four custom plugin implementations covering every extension
  point in the scheduling pipeline:
  • Filter — ModelAffinityFilter keeps only endpoints serving the requested model.
  • Scorer — LeastLoadedScorer prefers endpoints with more spare model capacity.
  • Picker — TopKRandomPicker selects randomly from the top-K highest-scored candidates.
  • ProfileHandler — LoggingProfileHandler adds tracing output to the scheduling lifecycle.
  • plugins/register.go — Centralised RegisterAllPlugins() following the same
    explicit-registration pattern as llm-d-inference-scheduler.
• examples/custom-epp/ — Shows how to build a production EPP binary that
  embeds out-of-tree plugins (make build-example EXAMPLE=custom-epp).
• examples/README.md — Comprehensive developer guide covering:
  • Scheduling architecture (Filter → Score → Pick pipeline).
  • Step-by-step instructions for writing each plugin type.
  • How to build and deploy a custom EPP (in-tree and out-of-tree patterns).
  • YAML EndpointPickerConfig example referencing custom plugins.
  • Multi-EPP deployment with InferencePool selection diagram.
• Makefile — Two new targets:
  • make run-example EXAMPLE=<name> — run an example locally.
  • make build-example EXAMPLE=<name> — build an example binary to bin/.

Which issue(s) this PR fixes:

Fixes #856

Does this PR introduce a user-facing change?:

NONE

/cc @ahg-g

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[k8s-ci-robot]

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