docs: document Anthropic-compatible API endpoints

[apitokensale-admin]

Summary

Document how Aider can connect to third-party endpoints that implement the Anthropic Messages API by setting ANTHROPIC_API_BASE, with apiToken.sale as a tested example.

This uses Aider's existing anthropic/ model path and the current LiteLLM integration; no application code or provider-specific routing is added. This documentation contribution was AI-assisted and manually validated.

Changes

• explain that ANTHROPIC_API_BASE takes the provider root URL without /v1/messages
• add Mac/Linux and Windows environment examples
• show an explicit anthropic/claude-sonnet-4-6 launch command
• clarify that apiToken.sale is a third-party service and its keys are not Anthropic-issued

Validation

• git diff --check
• codespell aider/website/docs/llms/anthropic.md
• production Jekyll build with Ruby 3.3
• rendered callout, heading, code blocks, and link inspected in the generated HTML
• https://apitoken.sale returned HTTP 200

Proof of working

Tested from current Aider main (5dc9490b) with its pinned LiteLLM 1.82.3 and a provider-issued test key supplied only through the process environment:

• Aider v0.86.3.dev53 connected through https://api.apitoken.sale using anthropic/claude-sonnet-4-6
• streaming edit completed and Aider applied the requested change to a temporary file
• basic, streaming, and forced tool-use calls through the pinned LiteLLM dependency all completed successfully

No credentials, response contents, or temporary test files are included in this PR.

[apitokensale-admin]

@greptileai

[CLAassistant]

All committers have signed the CLA.