DOC Document making variants with different file extensions

[GuySartorelli]

Description

This PR has two commits:

1. Refactors file docs a little, so that there's a clear place for documentation about manipulating file content. I wasn't sure where to put the new docs so now there's a place that's clear for readers and maintainers alike.
2. Documents NEW Allow file variants with different extensions silverstripe-assets#585

Please avoid recommending changes for content that I have simply lifted from one page and shifted to another.

Issues

• Allow file variant with different extension silverstripe-assets#488

Pull request checklist

• The target branch is correct
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• All commits are relevant to the purpose of the PR (e.g. no TODO comments, unrelated rewording/restructuring, or arbitrary changes)
  • Small amounts of additional changes are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
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[GuySartorelli]

@michalkleiner Please stop suggesting rewording for content that I moved from one file to another. Changing that wording is not in scope.

[michalkleiner]

Reads well. Mostly just minor stylistic suggestions and typo fixes.

[michalkleiner]

@michalkleiner Please stop suggesting rewording for content that I moved from one file to another. Changing that wording is not in scope.

I reviewed the change as a whole. In my opinion if there are changes/tweaks that make sense to do to improve the overall quality of docs while we are touching the area, especially when they can be simply accepted in bulk via Github UI, I don't see that as a bad thing. Feel free to reject those if it bothers you too much.

[GuySartorelli]

I've made the changes that didn't require any thought, or which resolved potential confusion, or which were in scope.
I'm not making the rest of the suggested changes as they were out of scope and required some thought that I'd rather spend elsewhere.

[emteknetnz]

Will hold off merging in case anything comes up in peer review on silverstripe/silverstripe-assets#585 (review)