Add external snippet generated docs automation

[danielporterda]

Follow-on to #796. This pulls the external-snippet generated-docs automation out of the core generated-docs automation PR so it can be reviewed separately.

Scope:

• adds the external snippet source manifest for Canton, CN Quickstart, Daml, Daml Shell, DPM, Scribe, and Splice
• adds the per-source external snippet target runner for GitHub Actions
• registers external snippet generated-doc targets in the orchestrator
• keeps Canton snippets behind the heavy-runner opt-in and skips unavailable private Daml Shell/Scribe sources
• normalizes copied generated .mdx snippet output before committing generated PR branches
• preserves authored Splice common/** wrapper snippets while replacing generated snippet output

Validation:

• direnv exec . python3 -m pytest tests/test_generate_external_snippet_target.py tests/test_generate_external_snippets.py tests/test_update_generated_reference_prs.py tests/test_summarize_version_changes.py (45 passed)
• direnv exec . python3 -m py_compile scripts/generate_external_snippet_target.py scripts/generate_external_snippets.py scripts/update_generated_reference_prs.py scripts/summarize_version_changes.py
• git diff --cached --check