Update README.md

Rename welcome-js-recap.md to A00-welcome-js-recap.md

Rename useffect-and-data-fetching.md to D02-useffect-and-data-fetching.md

Rename styling.md to C04-styling.md

Rename styled-components.md to C04-styled-components.md

Rename spread-rest-operator.md to A04-spread-rest-operator.md

Rename setup-computer-environment.md to B01-setup-computer-environment.md

Rename routing.md to D01-routing.md

Rename redux.md to D09-redux.md

Rename passing-props.md to C03-passing-props.md

Rename modules-exports-and-imports.md to A03-modules-exports-and-imports.md

Rename jsx.md to B03-jsx.md

Rename hello-world.md to B02-hello-world.md

Rename forms.md to D04-forms.md

Rename forEach-map-filter-reduce-string-interpolation.md to A05-forEach-map-filter-reduce-string-interpolation.md