docs: update document url in README

[wu8685]

What type of PR is this?

docs

What this PR does / why we need it:

The doc url in readme is out of date.

Which issue(s) this PR fixes:

Fixes #42

Special notes for your reviewer:

Correct the url with https://www.kusionstack.io/ctrlmesh/intro/

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., design docs, usage docs, etc.:

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