Added an introduction page for DMI

[Catherine-monk]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  docs update

• What is the current behavior? (You can also link to an open issue here)
  update the docs of dmi introduction

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

[kubeedge-bot]

Welcome @Catherine-monk! It looks like this is your first PR to kubeedge/website 🎉

[kubeedge-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[Shelley-BaoYue]

cc @wbc6080

[kubeedge-bot]

@Catherine-monk: PR needs rebase.

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