docs: add Shipmail warehouse source

[jcoulaud]

Changes

• Add setup documentation for Shipmail as a PostHog warehouse source.
• Document the required Shipmail API scopes, available tables, and sync modes.
• Explain the analytics-safe message projection and privacy behavior.
• Companion source implementation: PostHog/posthog#75069.

This is a documentation-only change, so no screenshots are needed.

Checklist

• I've read the documentation style guide and the content style guide.
• I've used American English throughout.
• I've used relative URLs for internal links.
• I've checked the Vercel preview.
• If I moved a page, I've added a redirect. (Not applicable; no page was moved.)

[jcoulaud]

Maintainer handoff:

• Rebased onto master at 06825aa11 with 0 commits behind at push time. master has since advanced by 4 unrelated content/docs commits, so the PR is currently 4 commits behind without a conflict.
• Standardized the product name and sourceId to Shipmail.
• Verified the source/docs slug mapping, Markdown lint, and Prettier formatting locally with no errors.
• There are no unresolved review threads.

The Markdown, Vale, security, and preview workflows are waiting for fork-workflow approval, and this external contributor account cannot approve them or request reviewers. @PostHog/team-wizard-and-docs, could someone approve the workflows, check the preview, and review this PR? The source implementation is PostHog/posthog#75069.