Automate Splice release note updates

[danielporterda]

Parent: #749

Current state

`docs-main/global-synchronizer/release-notes/splice.mdx` is copied manually today.

Goal

Automate updates for this published release-note page.

Scope

• Source: TBD. Splice release notes are expected to change shape to match Canton, so this task should define the durable source and format instead of baking in the current copy path.
• Output: docs-main/global-synchronizer/release-notes/splice.mdx
• Route: /global-synchronizer/release-notes/splice

Acceptance criteria

• cf-docs has a repeatable command or workflow for refreshing this page.
• The process detects whether the upstream source changed before opening or updating a PR.
• The published route and page frontmatter are preserved.
• The PR body identifies the upstream source revision/version and summarizes changed release-note sections.