Revise CONTRIBUTING acceptance criteria for the current dataset

[andygrunwald]

Summary

The acceptance criteria in CONTRIBUTING.md predated Netflix / Amazon Prime / bpb entries and the `localized:` infrastructure. Several rules were factually wrong; this PR rewrites the affected rules to match what the catalogue actually accepts today.

What changed

• "Freely viewable" → "Watchable on a mainstream platform." Free ad-supported (YouTube), common subscriptions (Netflix, Amazon Prime Video), public-broadcaster libraries (bpb), and pay-per-rent on a major store are all in scope; obscure regional rentals and short festival exclusives are not. The clause now mentions the `links:` map so contributors know where to declare every platform.
• "Educational content" examples extended to include content moderation and surveillance, matching The Cleaners / The Great Hack / Terms and Conditions May Apply.
• "Accessible in English" is now an explicit clause, with a pointer to `localized:` for alternate-language uploads of the same work.
• New out-of-scope bullet excluding PURELY fictional movies / TV series. Biopics and dramatised true stories (The Social Network, Pirates of Silicon Valley) stay in scope under `type: Movie`; only fully invented plots (Mr. Robot, Halt and Catch Fire, cyber-thrillers) are out.

What did NOT change

• The four positive criteria. They still describe the catalogue accurately.
• The 15-minute length floor.
• The "published more than 2 weeks ago" clause.

Audit against current entries

Every catalogued entry (32) passes the new rules. Spot checks:

• The Great Hack (Netflix) — passes "Watchable on a mainstream platform" via the Netflix subscription clause.
• The Cleaners (bpb) — passes via the public-broadcaster clause.
• Lo and Behold — top-level YouTube link satisfies the platform rule; the Amazon Prime Video German upload lives in `localized.de.links` per the "alternate-language uploads" guidance.

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