feat: add the docs for 'kcl.mod'.

[zong-zhe]

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

• N
• Y

fix issue #132

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

README-zh.md
README.md
docs/kcl_mod-zh.md
docs/kcl_mod.md

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

• 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 5863544686

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 2 files lost coverage.
• Overall coverage remained the same at 39.835%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/cmd/cmd_run.go	9	0%
pkg/cmd/cmd_pull.go	12	0%

Totals
Change from base Build 5722318220:	0.0%
Covered Lines:	1011
Relevant Lines:	2538

---

💛 - Coveralls

[Peefy]

LGTM