fix(docs): align fallback provider config examples

[aqilaziz]

Summary

• update fallback docs to use the current top-level fallback_providers list as the primary config shape
• keep fallback_model documented only as the legacy/back-compat shape
• align the fallback coverage table with current cron and subagent inheritance behavior

Verification

• git diff --check
• rg "fallback_model:\\s*$|do not support a fallback|no automatic fallback|subagents do not inherit" website/docs/user-guide/features/fallback-providers.md website/docs/user-guide/configuration.md

Note: pnpm --dir website run typecheck and pnpm --dir website run lint:diagrams could not run in this fresh clone because website/node_modules is not installed (tsc / ascii-guard not found).

Closes #19691

[alt-glitch]

Duplicate of #23163 and #23164 which already address the same files and issue (#19691). Those PRs are open and cover the same scope (aligning fallback docs with fallback_providers config shape).

[aqilaziz]

Thanks for flagging this. I missed #23163 and #23164 when opening this.

This PR does overlap with those two PRs on the fallback_providers config-shape cleanup. The main difference is that it folds both docs files into one change and also updates the fallback coverage text for cron jobs and delegated subagents to match current runtime/tests:

• cron/scheduler.py reads fallback_providers / legacy fallback_model and passes it into AIAgent(fallback_model=...).
• tools/delegate_tool.py passes the parent _fallback_chain into child agents, with coverage in tests/tools/test_delegate.py.

If maintainers prefer the older split PRs, I am fine closing this. If a single PR that also fixes the cron/delegation docs drift is useful, this one is green and mergeable.