docs: Promote cloud guides into a Concepts section

[developerjamiu]

Summary

• Adds a Concepts section to the Cloud docs by promoting the Logs, Passwords, Custom domains, and Database guides into concept pages, and introducing a Deployments concept that didn't exist before.
• Each concept covers overview, operations, and constraints in one page, so readers find one canonical page per capability instead of scattered how-tos.
• Removes the four corresponding guides plus the orphaned reference/02-logging.md; old URLs redirect to the new concept URLs via docusaurus.config.js.

Test plan

• npm run build passes locally (verified).
• Navigate the Cloud sidebar and confirm the new Concepts section shows: Deployments, Logs, Passwords/secrets/env vars, Custom domains, Database.
• Open each concept page and confirm operations, code examples, and cross-links resolve.
• Visit the old URLs (/cloud/guides/logs, /cloud/guides/passwords, /cloud/guides/custom-domains, /cloud/guides/database, /cloud/reference/logging) and confirm each redirects to the matching concept.
• Spot-check that Access the database content still surfaces (now under the Database concept) including the GUI client suggestions.

Out of scope

• The Concepts section currently inherits the Get Started sidebar icon (sidebar-icon-getting-started). A distinct icon is being introduced in a separate PR.
• Migration content for serverpod_auth → serverpod_auth_idp is tracked in docs: Document serverpod_auth_email setup and migrating from serverpod_auth to the new provider using serverpod_auth_migration #302 / docs: Add new guide for migrating email and Google from legacy auth to Serverpod 3 auth #445 / docs: Migrate-from-serverpod_auth guide for modular auth (fixes #3663) #477 and is not part of this PR.

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