tools: add doc generate command

[amyXia1994]

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

• N
• Y

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

cmds/kcl-go/command/cmd_doc.go
pkg/tools/doc/doc_test.go
pkg/tools/doc/kdoc.go
pkg/tools/doc/templates/schema.gotmpl
go.mod
go.sum

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

tools: add doc generate command

• the doc subcommand is now set to Hidden and will be turned on when the feature is ready to be released
• this PR adds the kcl-go doc generate command and implements the process from cli input to render markdown document to the target output directory. The more complete functionalities(such as starting a local documentation server, HTML/openapi format output, and the document template refine, ...) will be improved later.

• 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 5746479996

• 17 of 100 (17.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.3%) to 58.468%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/tools/doc/kdoc.go	17	100	17.0%

Totals
Change from base Build 5736506217:	-1.3%
Covered Lines:	1923
Relevant Lines:	3289

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💛 - Coveralls

[Peefy]

LGTM