Fix broken Mixpanel source anchor link + clean up a bunch of stuff

[pwseg]

Proposed changes

• Updated the broken link #group-using-device-mode to [Identify traits list](#identify). This is part of the migration cleanup work.
• The original anchor (#group-using-device-mode) referenced a legacy section that no longer exists. Its content (the Mixpanel special traits mapping table) now appears under the Identify section.
• While I was in there, I noticed... a bunch of style fixes. So I took care of them.
• I also reworded a bunch of weird/repetitive/unclear phrasing.
• Converted an HTML table to Markdown.
• Cleaned up some Swift code samples that weren't formatted correctly.

Merge timing

• ASAP once approved

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