Update intro.md

[benissimo]

docs: correct the definition of KCL.

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):

Documentation fix.

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

1. It is not true that KCL stands for "Kubernetes Configuration Language". That gives the misleading impression KCL is just for Kubernetes
2. Instead, some mention should be given to the true origin of the name, a reference to Kusion.
3. But it is important to clarify that it is now independent of Kusion.

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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