Update release checklist for main and dev-v3

[cookesan]

Summary

• document the current Helm branch split: main for Helm v4 and dev-v3 for Helm v3 maintenance
• clarify that Helm v3 fixes are backported to dev-v3 before a v3 patch release is cut
• update the docs release checklist to reference the current Docusaurus files instead of the old config.toml workflow

Why

The current release checklist still reflects an older docs setup and does not mention the active dev-v3 maintenance branch, which makes the release instructions harder to follow for current Helm maintainers.

Validation

• git diff --check
• yarn install --frozen-lockfile
• yarn build (passes; existing unrelated broken-anchor warnings remain in translated v3 pages)

Fixes #1917