docs(platform): add API-based database creation guide

[ArnobKumarSaha]

Summary

• Add create-database/using-api/ docs covering the API flow for provisioning databases (equivalent to the Create Database wizard): _index.md overview of the generic 3-call flow, plus mongodb.md and postgres.md engine-specific walkthroughs with example curl calls and request/response JSON.
• Link the new API guide from create-database/_index.md.

Test plan

• Verified all new/changed internal links with liche -p -h -l -s per repo CI convention

[github-actions]

Visit the preview URL for this PR (updated for commit 985a784):

https://kubedb-v2-hugo--pr149-arnob-api-create-db-nirrvhi6.web.app

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