docs: document external ALCops configuration

[someC0d3r]

Summary

• Documents Extends.Source for centrally maintained ALCops configurations.
• Describes supported HTTP(S) and absolute local-file sources.
• Clarifies that HTTP(S) URLs containing embedded credentials are rejected before network access.
• Documents precedence, nested-object merging, array replacement, caching, timeout, fallback behavior, and the single-level inheritance restriction.

Related implementation

• Analyzer draft PR: feat: support remote ALCops configuration Analyzers#500
• Original proposal: Support Inheriting ALCops Configuration from Remote "alcops.json" Configurations Analyzers#483

Validation

• Hugo Extended production build completed successfully.
• 163 pages generated with no build errors.
• Existing upstream Hugo deprecation warnings are unchanged.

Codex disclosure

This draft PR was generated with OpenAI Codex.

Model: GPT-5 family. The exact runtime model identifier and reasoning-effort setting were not exposed in this session.
Author note: 5.6 Sol, Reasoning: Very High (4/5)

Reasoning approach: the implementation behavior and repository documentation conventions were analyzed, the configuration guide was updated to match the tested analyzer semantics, and the complete Hugo site was built locally. Private chain-of-thought is not included.