Guide developers writing experience descriptions

[yawboakye]

Summary

• add a concise hint for writing contribution-focused experience descriptions
• add a four-item self-review checklist beside the Markdown editor
• keep checklist state browser-only by omitting submitted field names
• cover both new and edit experience forms
• omit canned examples and defer static scoring to a separate change

This is a focused follow-up to #446.

Screenshot

Testing

• bundle exec standardrb app/components/forms/experience_description_input.rb packages/developers/app/definitions/developers/experience_definition.rb test/integration/developer_portal_test.rb
• RUBYOPT=-r/tmp/connect_vips_boot_shim.rb bin/rails test test/integration/developer_portal_test.rb
• npx -y @yarnpkg/cli-dist@4.18.1 build:css
• browser-verified the hint, checklist layout, checkbox interaction, and absence of examples/scoring