LIN-2577 document automatic cohort sync alongside manual upload

[tashviks]

Splits features/remarketing-audience-upload into Automatic sync and Manual upload, leading with the automatic path.

A cohort is a live rule, so a CSV export describes who matched on the day it was downloaded — and re-uploading a file only ever adds people, never removes the ones who stopped matching. The page now opens with that distinction instead of treating both routes as equivalent.

What the automatic section covers

Setup, the status chips, and the things that will otherwise generate support tickets — all of which surfaced while testing this against a live Google Ads account:

• Google applies uploads asynchronously (up to 48h) and hides audiences below roughly 100 matched members for Display / 1,000 for Search. A perfectly working sync shows a size of 0 for a small cohort, which reads as failure.
• Removal happens by membership expiry, not deletion. Left at the 540-day maximum, nobody is ever removed. The duration has to be set near the sync cadence or the audience is effectively append-only.
• Rule-based audiences cannot receive uploads. Pointing a destination at something like "All Converters" fails every time.
• Phone numbers without a country code are dropped rather than guessed at — that is what Default country is for.

The existing manual walkthrough is unchanged apart from heading levels.

Not in this PR

• Screenshots. Three spots are marked with MDX comments in the Set up the sync steps. I could not capture them — the local dashboard session had expired and I did not want to interrupt for a login.
• Scheduled syncs. The page describes syncing as ongoing, which is true of Sync now today but the nightly sweep is not yet wired up in ops. Worth confirming that ships before this page publishes, or the "stays current" claim overpromises.

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