[Post release v0.5.0] Update release doc

[kevin85421]

Why are these changes needed?

• Update the release document and improve the release process based on the learnings from v0.5.0.
• Update the format from 1,2,3 to headings because the former will make some issues for mkdocs.

Related issue number

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(

I checked both markdown and mkdocs.

• mkdocs

  # path: kuberay/
  docker run --rm -it -p 8000:8000 -v ${PWD}:/docs squidfunk/mkdocs-material

[architkulkarni]

Looks awesome!!

[kevin85421]

@architkulkarni I add two new commits for post-release PRs. Thanks!

[kevin85421]

This PR only updates documents. It is unrelated to the CI failure. Merge this PR.