docs(fallback): document the actual config shape (top-level fallback_providers) (#19691)

[0xsir0000]

Summary

The fallback-providers user-guide page was leading with the legacy `fallback_model:` (singular dict) shape across nine YAML examples, the intro paragraph, the custom-endpoint section, the auxiliary cross-reference, and the summary table. The CLI-reference page already showed the correct top-level `fallback_providers:` list shape, and `hermes_cli/fallback_cmd.py` + `cli.py` only persist to that key — so users hand-editing config from the docs ended up with a key the writer never updates and the reader treats as legacy back-compat only.

The intro line under `hermes fallback` also wrongly said the chain persists under `model.fallback_providers` — the writer has never put it nested under `model:`; the deep-merge config loader silently ignored the misplaced key, producing the confusing failover behaviour the issue describes.

Fix

Rewrite the page to:

• lead with `fallback_providers:` (top-level list) as the primary shape,
• add a worked "multiple fallbacks tried in order" example so the list format's value is obvious,
• update the custom-endpoint, auxiliary, and summary-table snippets to use the same shape,
• correct the `hermes fallback` intro to "top-level `fallback_providers:` list ... not nested under `model:`",
• keep the `fallback_providers` vs `fallback_model` admonition (now flipped — current shape first, legacy shape second), and refresh the env-var tip and "main provider only valid in ..." prose to mention both keys.

Also touch `configuration.md` (the auxiliary-section cross-reference and two `"main"`-only-valid-in-X notes) so it stops pointing readers back at the legacy key.

Verification

• ✅ `hermes_cli/fallback_cmd.py` lines 36–54 confirm: write to `fallback_providers`, read both `fallback_providers` (preferred) + legacy `fallback_model`.
• ✅ `cli.py` line 2224 confirms the same precedence.
• ✅ The CLI reference (`reference/cli-commands.md:1110`) and env-vars reference (`reference/environment-variables.md:523-531`) were already correct; this PR aligns the user-guide page with them.

Fixes #19691

[alt-glitch]

Duplicate of #19790 — both fix #19691 by correcting the fallback provider config shape in the same doc files (configuration.md, fallback-providers.md). Note that #20382 (merged) already fixed the CLI reference docs portion.