update doc render: avoid multiple index.md; remove redundant schema prefix

[amyXia1994]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y
  related to [Feature] KCL doc tool: extract, generate document of KCL packages #139

2. What is the scope of this PR (e.g. component or file name):

pkg/tools/gen/gendoc.go

pkg/tools/gen/templates/doc/packageDoc.gotmpl
pkg/tools/gen/templates/doc/schemaDoc.gotmpl

pkg/tools/gen/testdata/doc/k8s/...
pkg/tools/gen/testdata/doc/pkg/...
pkg/tools/gen/testdata/doc/reimport/...

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

update doc render:

• avoid multiple index.md with the same file name
• remove redundant schema prefix

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

[coveralls]

Pull Request Test Coverage Report for Build 6023242803

• 28 of 30 (93.33%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.08%) to 64.622%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/tools/gen/gendoc.go	28	30	93.33%

Files with Coverage Reduction	New Missed Lines	%
pkg/tools/gen/gendoc.go	1	68.48%

Totals
Change from base Build 6012221066:	0.08%
Covered Lines:	2539
Relevant Lines:	3929

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💛 - Coveralls