docs: add API Token Sale Anthropic endpoint

[apitokensale-admin]

Description

Closes #13048.

Documents Continue’s existing custom Anthropic apiBase support using API Token Sale as a concrete Anthropic-compatible example. The guide includes YAML and legacy JSON configurations, the required trailing /v1/, a provider-issued key placeholder, and an explicit third-party disclaimer.

This is documentation-only; no provider or runtime code changes are needed. The documentation was drafted with AI assistance and verified against Continue’s current Anthropic implementation and a live endpoint.

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Checklist

• I’ve read the contributing guide
• The relevant docs have been updated
• The relevant existing tests were run

Screen recording or screenshot

Not applicable — documentation-only change.

Tests

• Prettier check for the changed MDX: passed
• docs-site production build: passed (342 static pages)
• Internal package build order from scripts/build-packages.js: passed
• core/llm/llms/Anthropic.vitest.ts: 8/8 passed
• Live Continue Anthropic handler: basic response, SSE/usage, and forced tool use passed with claude-sonnet-4-6
• Rendered page contains the heading, configuration tabs, API host, disclaimer, and homepage link

An additional docs-site tsc:check reports the pre-existing MapIterator target error in app/[[...slug]]/page.tsx:42; the documented GitHub Pages workflow runs the successful production build and does not run this extra command.

[github-actions]

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[apitokensale-admin]

@greptileai please review