docs: add file.abs and file.exists documents

[Peefy]

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

• N
• Y

re kcl-lang/kcl#1157

docs: add file.abs and file.exists documents

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

• docs/reference/model/file.md
• i18n/zh-CN/docusaurus-plugin-content-docs/current/reference/model/file.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

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Name	Status	Preview	Comments	Updated (UTC)
kcl-lang-io	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 25, 2024 4:12am