Promote Web Editor preview to its own topic

What does this MR do?

doc/user/project/repository/web_editor.md documented the Preview tab as Markdown-only, in an H4 topic nested under editing a file. That wasn't accurate. The tab renders other markup formats as markup and shows a diff for everything else. It also applies when creating a file, not just editing one.

This MR promotes the topic to ### Preview a file, an H3 task topic under "Manage files", and rewrites it to describe what actually renders. It also broadens a page-intro bullet that referred only to live Markdown previews.

Related issues

• Depends on !253963 (merged), which adds Write and Preview tabs to the new file page, and must merge first.
• Related feature request: #15501.