docs: add AdCP vs RTB one-minute explainer page

[bokelley]

Closes #2573

Adds a top-of-funnel "AdCP vs RTB" comparison page at /docs/adcp-vs-rtb, driven by Addie weekly insights showing ~90 foundational "What is AdCP / how does it differ from RTB?" questions per week. The existing adcp-vs-openrtb.mdx targets implementers; this page targets the pre-enrollment, discovery-mode audience.

Changes

• docs/adcp-vs-rtb.mdx — new one-minute explainer: 4-row comparison table, "how they fit together" section (before/after the auction framing), inline FAQ (no accordions so the content is skim-visible), Academy Basics + Ask Addie CTAs
• docs.json — nav entry in both 3.0 and latest version groups, immediately after docs/intro
• docs/intro.mdx — Card added to "Where do you want to start?" CardGroup (updated to cols={3})
• docs/building/understanding/adcp-vs-openrtb.mdx — <Note> cross-link callout at the top routing discovery-mode readers to the new page
• .changeset/adcp-vs-rtb-comparison-page.md — empty changeset (docs-only, no protocol impact)

Non-breaking justification

Adds new MDX page and nav entries; no existing pages or schema fields modified in a breaking way. Homepage card and technical-page cross-link are purely additive.

Pre-PR review

• code-reviewer: approved — no CI-impacting blockers; nits surfaced: og:title redundancy (fixed), pronoun agreement (fixed), cols={2}→cols={3} orphan card (fixed), TMP link path convention (fixed)
• docs-expert: approved — factual accuracy confirmed against FAQ and technical page; "layers" framing replaced with "before/after the auction" throughout; "Scope"+"Operations" table row overlap resolved by collapsing to "What it handles"; Note callout framing updated from "short version" to audience-routing

Nits surfaced (not fixed — for reviewer awareness):

• "Academy Basics (free)" card title uses parenthetical — can remove (free) qualifier if preferred
• <Note> on adcp-vs-openrtb.mdx uses "coming from real-time bidding" — reviewers may prefer different phrasing

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See #3121
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Session: https://claude.ai/code/cse_01HULrqPhn8kLM76nSmcpj4d

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[bokelley]

Closing — the data behind this PR doesn't hold up.

The "~90 foundational AdCP-vs-RTB questions per week" cited in #2573 is an LLM-extrapolated theme.count from server/src/addie/services/conversation-insights-builder.ts (the prompt at line 354, 363 explicitly asks Claude to estimate frequency across all threads from a sample), and the sample is biased toward escalations and low ratings (line 254-257). It is not a SQL count.

I queried the prod admin API for all Apr 11–18 threads matching "rtb" anywhere in message content (the exact window #2573 cites) — 103 threads. Of those:

Count	First user message	Source
22	Start module A1	Sage Academy curriculum CTA
14	What is AdCP and how does it work?	chat.html welcome button "Learn about AdCP"
13	What can you do? What kinds of things can I ask you about?	chat.html welcome button
12	How is agentic advertising different from programmatic, and why does it matter?	chat.html welcome button "AdCP vs programmatic"
10	I'd like to start learning AdCP in the Academy…	Academy CTA
3	Start module A2/A3/B1	Sage curriculum

72% (74 of 103) start with a canned CTA chip, not a typed question. The CTA chips are at server/public/chat.html:2547-2552. "rtb" matched in those threads because Addie/Sage mentions OpenRTB/RTB in its response, not because the user asked.

Of the 28% (29) genuinely typed messages, only a handful are anywhere near "AdCP vs RTB?" as a discovery question. The rest are mid-curriculum learners or different topics ("how does AdCP compare to IAB AAMP?", "Where does a DSP fit?", "what commercial agreements do I need", etc.).

There is no measurable cohort of pre-enrollment "AdCP vs RTB?" askers leaving without enrolling. The existing docs/building/understanding/adcp-vs-openrtb.mdx plus Sage's A3 module already serve the audiences that actually ask.

Closing this PR. Closing #2573 (the source issue) for the same reason.

Separately filing an issue on the weekly-insights pipeline — including CTA-chip-initiated threads in theme frequency means any homepage button essentially manufactures its own demand signal.

The IAB Tech Lab and AAMP questions are real and recurring, so I tightened the FAQ entries on both in a separate change.

[bokelley]

Acknowledged — the closure makes sense given the data. The 72% CTA-chip contamination explains why the raw RTB-thread count looked like organic demand; the existing adcp-vs-openrtb.mdx + Sage A3 already cover the audience that actually shows up with the question.

Good catch on the insights pipeline — worth tracking so theme frequencies don't manufacture their own demand signal going forward.

Session: https://claude.ai/code/session_017GN4Ztqi4J9aqeKBAVW1oS

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[bokelley]

Noted — no action needed here. Closed PR stands.

Triaged by Claude Code. Session: https://claude.ai/code/session_01Qok8hB7ZHv1HCwadpsW51v

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