doc, child_process: clean up Windows example

[reillylm]

In #53616, we added MJS snippets for the page. In the section about Windows batch files, we also combined two previous snippets who demonstrated the three ways of using spawn/exec described in the above paragraph.

The first snippet showed the spawn of cmd.exe way:

node/doc/api/child_process.md

Lines 112 to 114 in dc74f17

	// On Windows Only...
	const { spawn } = require('node:child_process');
	const bat = spawn('cmd.exe', ['/c', 'my.bat']);

The other snippet showed the exec and spawn with shell: true ways:

node/doc/api/child_process.md

Lines 130 to 132 in dc74f17

	// OR...
	const { exec, spawn } = require('node:child_process');
	exec('my.bat', (err, stdout, stderr) => {

node/doc/api/child_process.md

Line 141 in dc74f17

const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });

This makes these changes:

• remove the // OR... comment, since we only have one snippet now, so there's nothing to "or" with :)
• bring back the spawn('cmd.exe') example, which got lost in the shuffle
• reorder the examples to match the way they're presented in the paragraph
• remove the exec callback body, since the focus of the snippet is just the invocations of spawn/exec, not the usage of exec specifically
• split the prose of the three ways into a list so it's easier to read