[WIP]: cloud: add concept docs

[qiancai]

What is changed, added or deleted? (Required)

Add a series of key concept documents to make TiDB Cloud documentation more intuitive and helpful for new users

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[shhdgit]

tidb-cloud/architecture-cocepts.md -> tidb-cloud/architecture-concepts.md

[ti-chi-bot]

@qiancai: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	06dcfca	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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