[codex] Add external snippet provenance comments

[danielporterda]

What changed

Adds a searchable source-snippet HTML comment to generated external snippet MDX files. The comment records the source repo, source path, source commit hash, snippet name, and extraction location.

The generator keeps import-leading MDX valid by placing the provenance comment after the import block when a generated snippet starts with import statements.

This refresh also updates the wrapper copy target to the current checked-in tree under docs-main/snippets/external, documents that location in the update workflow, and adds provenance comments to existing checked-in snippet files that still map cleanly to current extraction config.

Why

This creates a stable text-search join between generated snippet files and the source repository state that produced them, which is the first step toward auditing orphaned or stale external snippets.

Refresh and run status

• Rebased onto latest main at 25425b81c5c7d7948a14dd7d02faeb381235f8ec.
• Added provenance comments to 665 existing config-mapped snippet files.
• Left 139 older checked-in snippet files unchanged because they are not represented by the current extraction config; adding synthetic provenance there would be misleading.
• Extraction succeeded for cn-quickstart, daml-shell, scribe, splice, and wallet-gateway.
• Extraction failed for stale source/config entries in canton, daml, and dpm; those failures happen before a trustworthy full corpus copy can be made.

Validation

• direnv exec . node --check scripts/generateOutputDocs.js
• direnv exec . node --check scripts/helpers/generateOutputDocs.js
• direnv exec . python3 -m py_compile scripts/generate_external_snippets.py
• direnv exec . npm run generate:external-snippets -- --list
• git diff --check