docs: consolidate obsolete pages and label retained legacy references

[enyst]

Follow-up to the orphan-page audit in #815, stacked on #816.

Five superseded pages are removed with redirects: the old About page, debugging guide, development overview, First Projects, and SDK overview. Beginner prompts move into the navigable Tutorial Library. Current inbound links and older redirect aliases now point directly to their replacements.

Five references remain accessible with legacy/archive notices and noindex: true: CodeAct agents, the old evaluation harness, Socket.IO protocol, named TOML LLM configs, and the mixed environment-variable reference. Current Canvas runtime settings remain in Canvas Development, as preserved by #815. The SDK LLM architecture page now points to the SDK profile store. The three unverified “Coming soon” Cloud integration pages are left intact.

This intentionally does not present Canvas architecture as a full replacement wire-protocol guide, or benchmark runners as a generic harness-authoring guide.

Stack: #815 → #816 → this PR. GitHub cannot include fork-based #815 in a native stack. #816 targets an upstream copy of its exact commit; this PR targets #816. Keep both follow-ups draft until #815 merges and the stack is rebased/retargeted to main.

Source verification:

• Current Canvas development guide replaces the former monorepo development paths.
• Native WebSocket construction and event socket URL support marking the old Socket.IO instructions as legacy.
• Canvas runtime settings forwarding.
• Conversation.send_message updates state and returns None, contradicting the removed SDK overview's immutable add_message() example.
• Current benchmark setup and SDK-based runner.

Validation: all five deleted URLs have direct redirects to existing pages; redirect sources are unique; removed pages are absent from navigation; added internal links and anchors resolve; changed YAML/JSON parse; retained legacy pages have boolean noindex; git diff --check passes. Local Mintlify rendering is blocked by the previously reported uv_interface_addresses environment error.