docs: document X-Trickster-Result header

[houyuwushang]

Description

Related to #492.

Adds a dedicated X-Trickster-Result documentation page covering:

• the header field format (engine, status, fetched, ffstatus, failed);
• cache/result status meanings, including proxy-only;
• common reasons a request can be proxied without cache use;
• Fast Forward status values.

Also links the cache status reference to the new page and fills in status values that exist in code but were missing from the docs table.

Validation:

• go test ./pkg/proxy/headers ./pkg/cache/status -count=1
• git diff --check

Type of Change

• • Bug fix
• • New feature
• • Optimization
• • Test coverage
• • Documentation
• • Infrastructure

AI Disclosure

• • This contribution DOES NOT include AI-generated changes
• • This contribution DOES include AI-generated changes, and I have reviewed the relevant contributing guidelines.