docs: add kusion module design doc

[SparkYuan]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #643

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Additional documentation e.g., design docs, usage docs, etc.:

[coveralls]

Pull Request Test Coverage Report for Build 7969997014

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 17 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.1%) to 76.489%

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Files with Coverage Reduction	New Missed Lines	%
pkg/engine/operation/parser/spec_parser.go	4	56.1%
pkg/engine/operation/parser/parser.go	6	75.61%
pkg/cmd/destroy/options.go	7	81.86%

Totals
Change from base Build 7968280233:	-0.1%
Covered Lines:	8784
Relevant Lines:	11484

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💛 - Coveralls