Provide guidance on "Programmatically creating a file"

[maxime-rainville]

Pages affected

• File management

Description

"Programmatically creating a file" is one of the most common use case. We don't give you clear guidance on how to do it and there's several bits that can trip you.

Acceptance criteria

• A clear concise code example on how to "Programmatically create a file" is provided.
• The doc calls out the risks of creating the file in the wrong stage.
• The code example explicitly sets the stage.
• The doc explains how to generate thumbnails when programmatically creating an image.

Validations

• [X] Check that there isn't already an issue that reports the same problem

[maxime-rainville]

Maybe we want to revisit that entire doc page, while we are at it. It goes into a lot of esoteric topics and misses some rather obvious ones.

[maxime-rainville]

We reviewed this doc for the file variant job. It may not be relevant anymore.

[maxime-rainville]

This PR who is attached to a different job will solve this problem: https://github.com/silverstripe/developer-docs/pull/441