docs: add release guide and align chart publishing

[Asish Kumar (officialasishkumar)]

Description of your changes

This PR adds a maintainer-facing RELEASE.md, links it from CONTRIBUTING.md, and aligns the chart publishing workflow with the tagged release path so the documented behavior matches the actual release automation.

Fixes #536

I have:

• Associated this change with a known KubeFleet Issue (Bug, Feature, etc).
• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

• make reviewable
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/chart.yml"); puts "chart.yml ok"'
• Simulated the publish-github-pages metadata rewrite step on temporary copies of both Chart.yaml files to verify the workflow rewrites version and appVersion to the release values.

Special notes for your reviewer

• This PR intentionally updates .github/workflows/chart.yml in addition to the docs so the release guide matches the tagged release behavior for both OCI charts and the gh-pages chart repository content.
• The guide distinguishes between verifying the gh-pages branch content and verifying the public GitHub Pages Helm endpoint, which still depends on GitHub Pages being enabled for the repository.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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