Docs preview CI report links the site root and points at /handbook/engineering/

Problem

Someone who adds a page under docs/published/docs/ gets a preview link to the site root, plus an instruction to open /handbook/engineering/. They have to work out their own page's URL, and the path they are pointed at has nothing to do with what they changed.

Current output

The docs-preview section of the CI report reads:

Docs from this PR will be published at posthog.com.

Project	Preview	Updated (UTC)
posthog.com	Open preview	...

The preview should be ready in about 10 minutes. Open the preview at /handbook/engineering/.

Example on #83116, which adds docs/published/docs/data/test-accounts.mdx. The page is at /docs/data/test-accounts, which the section never mentions.

Cause

The hint is hardcoded, and predates product docs living in this repo. docs/published/ held only handbook content when it was written. It now also publishes pages under docs/published/docs/, which land on /docs/....

Suggested fix

.github/scripts/post-docs-preview-section.mjs runs where the changed files are known. The URL is already a pure function of the path, per docs/README.md: strip the docs/published/ prefix. Listing a direct link per changed page would remove the guesswork, and would also make it obvious when a page has landed at an unintended URL.

Start with .github/scripts/post-docs-preview-section.mjs and read the URL rules in docs/README.md. Trace how changed files under docs/published/ reach the docs-preview section, then verify the report gives each changed page its direct preview URL instead of the unrelated site root and handbook path.