Support building data from a single file

Hey, I'm not sure if this is possible already, but I would like to be able to build data from a single file

Example ... `data/projects.json`

There I would have something like:
```json
[
{
"title": "Blueprint",
"description": "Dashboard for creating Android icon packs",
"icon": "/static/images/projects/android/blueprint.png",
"preview": "/static/images/projects/android/blueprint-preview.png",
"link": "https://github.com/jahirfiquitiva/Blueprint/",
"color": "#4d8af0",
"tag": "android",
"stack": [
"android",
"kotlin",
"material design"
]
},
{
"title": "Frames",
"description": "Dashboard for creating Android wallpapers apps",
"icon": "/static/images/projects/android/frames.png",
"preview": "/static/images/projects/android/frames-preview.png",
"link": "https://github.com/jahirfiquitiva/Frames/",
"color": "#35b4ef",
"tag": "android",
"stack": [
"android",
"kotlin",
"material design"
]
}
]
```

And `contentlayer` would be able to return each item as a document of type project, or something like that

Beginne damit, die aktuellen Build-Einstiegspunkte von Contentlayer zu überprüfen und zu untersuchen, wie die Beispieldatei data/projects.json gefunden werden würde. Bestimme, wie das JSON-Array auf Projektdokumente abgebildet werden soll und welche Form das zurückgegebene Dokument haben muss. Fertig ist die Aufgabe, wenn jedes Element als Projektdokument verwendet werden kann.

Vom Indexierungsmodell aus dem Issue-Text verfasst.