add overview explanation about how CI/CD github workflow works.

[fujitatomoya]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Which issue(s) this PR fixes:

Fixes #

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

It simply adds the overview explanation how github workflow goes in kubeedge project.

• What is the current behavior? (You can also link to an open issue here)

No documentation available.

• What is the new behavior (if this is a feature change)?

Overview is explained.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information:

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[gemini-code-assist]

Code Review

This pull request introduces a new developer documentation file (docs/developer/ci_cd.md) that comprehensively details the CI/CD workflows for KubeEdge, including core CI, docs-only CI, code quality, security, scheduled compatibility tests, and release workflows. The review feedback points out a typo in the job name release-assests which should be corrected to release-assets.

[fujitatomoya]

doc build is supposed to fail because of #804. we need to rebase this against it.