docs: Open every Cloud concept page with a reason to care

[developerjamiu]

Summary

Every Cloud Concept page now opens with a concrete reason a Builder would care, before any mechanics. Surfaced during the Deployment hooks review on #592, where the first draft opened with "scripts at fixed points during a deploy" instead of "build a Flutter web client, run database migrations, send a Slack notification on release".

The audit covers seven pages: Deployments, Logs, Passwords/secrets/env vars, Custom domains, Database, Personal access tokens, Deployment hooks. The concept template at `.github/doc-templates/concept.md` is updated to make the rule explicit and the publishing checklist matches.

Two small cleanups landed in the same pass because the new opener made them visible:

• Custom domains had a redundant restatement of TLS/DNS-verification two paragraphs below the opener. Dropped.
• Database had "Cloud manages the infrastructure for you" duplicating "so you don't have to set up or maintain Postgres yourself" from the previous paragraph. Dropped.

Test plan

• `npm run build` passes in CI.
• Each opener leads with a use case or stake before naming the capability.
• Read each concept page top to bottom and confirm the new opener flows into the next paragraph or section without redundancy.