Reorganise and document release script entry points (#42774)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42774

Reorganise release scripts so that command entry points are grouped based on execution context, which also reflects dependencies between scripts.

Also:

- Document the current behaviours of these scripts.
- Relocate utils out of the root contents.
- Replace `exec` call to `set-rn-version` script with function import.

NOTE: `yarn trigger-react-native-release` (documented command in release process) is unchanged, since this is aliased from `package.json`.

```
├── releases
│   ├── templates/
│   ├── utils/
│   ├── remove-new-arch-flags.js
│   ├── set-rn-version.js
│   └── update-template-package.js
├── releases-ci
│   ├── prepare-package-for-release.js
│   └── publish-npm.js
└── releases-local
    └── trigger-react-native-release.js
```

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D53274341

fbshipit-source-id: eec2befc43e7a47fd821b2e2bcc818ddffbb6cf7

scripts/testing-utils.js

@@ -6,6 +6,7 @@
  *
  * @flow strict-local
  * @format
+ * @oncall react_native
  */
 
 'use strict';
@@ -18,7 +19,7 @@ const circleCIArtifactsUtils = require('./circle-ci-artifacts-utils.js');
 const {
   generateAndroidArtifacts,
   generateiOSArtifacts,
-} = require('./release-utils');
+} = require('./releases/utils/release-utils');
 const fs = require('fs');
 // $FlowIgnore[cannot-resolve-module]
 const {spawn} = require('node:child_process');