[docs] Add a scheduling overview with defaults to the Ray Core scheduling page

[dstrodtman]

Description

doc/source/ray-core/scheduling/index.rst carried a standing note that the page lacked an overview of the features and configs that affect scheduling, along with their default values.

Most of those defaults are already documented, but they're spread across the resources, labels, and placement group pages, so a reader has no single place to see what Ray does before any configuration.

This PR adds a "Scheduling at a glance" section that:

• Describes the two-step placement model: narrow to feasible nodes, then pick one.
• Tabulates each control with its default, linking to the page that documents it rather than restating the mechanism.
• Tabulates the three environment variables that tune the "DEFAULT" strategy.
• Calls out that the non-zero actor scheduling default means an actor needs a node with a free CPU to start, which is a recurring source of confusion.

It removes the standing note, since this addresses it. Happy to restore it if maintainers consider the gap only partly closed.

One genuine addition: RAY_scheduler_top_k_absolute was named on the page without its value. It defaults to 1 per src/ray/common/ray_config_def.h.

Related issues

None. The work is driven by the in-file note.

Additional information

Not a duplicate

Searched open PRs against ray-project/ray for "scheduling overview" and related area keywords. Nothing open touches this page.

Testing

Every default in the new tables was verified against source, not against other docs pages:

• The three scheduler environment variables come from src/ray/common/ray_config_def.h: scheduler_spread_threshold 0.5, scheduler_top_k_fraction 0.2, scheduler_top_k_absolute 1.
• The actor defaults were checked in python/ray/actor.py. A bare actor takes the simple branch, giving it 0 lifetime CPU and an actor_method_cpu of 1. Because method CPU is 1, actor_placement_resources becomes the lifetime resources with CPU incremented by 1. That's what makes "1 CPU for scheduling, 0 for running" true, and it's why the constants alone read as if they say the opposite.
• Node auto-detection and the task default come from scheduling/resources.rst; the default labels come from scheduling/labels.md.
• Every :ref: and :doc: target in the new section was verified to exist by grepping for its label definition.

Other checks:

• vale doc/source/ray-core/scheduling/index.rst — the added content introduces no new class of Vale error. The message-set diff against the pre-change file is empty.
• pre-commit run on the staged file — every hook reports "no files to check".
• A full Sphinx docs build was not run locally. Cross-reference resolution relies on CI.

Self-review findings already fixed

A critical pass before requesting review caught three problems in the first draft:

• Data locality was described as breaking ties for tasks. The page's own "DEFAULT" section says locality takes precedence over utilization scoring, so Ray prefers the locality node regardless of how utilized it is. "Tie-breaking" understated it.
• The gang placement row referred to bundles being scheduled independently outside a placement group, but bundles only exist inside one, so it described something that can't happen.
• A sentence said "both defaults above are non-zero" when the two actor defaults are 1 and 0, contradicting the row directly above it.

AI assistance

AI assistance was used to draft this section. Every default stated was read out of the source tree rather than recalled. It still needs a final human review pass, which is why this is opened as a draft.