Update files.md

[dewimagento]

Purpose of this pull request

(Handy and mostly unrelated tip:
I find the following bookmarklet handy for very wide code-reviews:

javascript:document.body.classList.toggle('full-width');

• putting that in your bookmarks menu or bookmark bar and clicking it while, reviewing git's diff page, makes the git diff take the whole width of the browser.]

This PR updates the files.md file:

• Fix "smart" unicode quotes throughout to correctly be the " and ' characters.
• Wrap lines to ~120 chars throughout.
• Add table of params for the Get files endpoint.
• Clarify the explanation of the multipart form upload.
• Remove all references to the DELETE file endpoint, no longer available to users (which I only finally got confirmation about just now, yay).

Affected DevDocs pages

https://devdocs.magento.com/guides/v2.3/marketplace/eqp/files.html

[devops-devdocs]

An admin must run tests on this PR before it can be merged.

[ghost]

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