Refresh: what-is-platform-engineering pillar for GSC/GEO gaps (#21038)

* Refresh: what-is-platform-engineering pillar for GSC/GEO gaps

Corrects the premise driving this refresh: the -20.8pp Profound figure
that originally motivated it was a one-prompt sample from a topic that
had already been expanded to 6 prompts on 2026-08-18, and Profound has
been unreachable (0 tools) since 2026-08-19, so it could not be
re-verified here. The refresh proceeds on Search Console evidence
instead: weekly impressions on this page roughly halved from mid-June
to mid-August, and average position slid from ~12-15 to 21-23 before
partially recovering, while the page still converts almost no clicks
or engaged GA4 sessions for its own head terms.

Changes:
- Answer-first bolded definition opens the page (was an unbolded
  two-sentence paragraph).
- New sections: how to build a platform engineering team, a tools
  category table, how to measure platform success, common myths, and
  a short how-do-you-become-a-platform-engineer note, closing gaps
  versus platformengineering.org, Red Hat, Humanitec, and Google Cloud.
- How is AI changing platform engineering rewritten for 2026 framing:
  agents as first-class platform tenants with RBAC/quotas (agent
  golden paths), and IaC's role as the auditable system of record
  agents reconcile against, per Microsoft Azure's 2026 agentic
  platform engineering analysis.
- Adds the reciprocal link to /blog/platform-engineering-agentic-era/
  inside the existing AI agents as a new class of platform consumer
  bullet, reproduced byte-identically to the text specified in PR
  #20949 (Platform Engineering in the Agentic Era) so the two
  branches merge cleanly. That post is not yet on master (PR #20949
  is still open); the link 404s until #20949 lands. #20949 should
  merge first, or together with this PR.
- Adds named, dated statistics from Gartner (already present), DORA
  2025, the Platform Engineering community's State of Platform
  Engineering Vol. 4 and State of AI in Platform Engineering 2025,
  Perforce's State of DevOps Report 2026, and Puppet's 2024 State of
  DevOps Report, plus a real attributed quote from Washington Trust
  Bank's Dennis Sauve already used in a linked case study.
- Cross-links internal hub-and-spoke siblings that already outrank
  this pillar for adjacent terms: the platform-engineering-idp guide,
  the CNCF maturity model post, what-is-an-internal-developer-platform,
  what-is-agentic-infrastructure, what-is-gitops, what-is-policy-as-code,
  and /docs/idp/.
- Verified via an isolated Hugo harness that the FAQPage schema picks
  up all 16 question-form headings with no @id collisions, and that
  every internal link resolves on master except the intentional
  pending one above.

Deliberately not chased: platform engineering services/consulting head
terms, which carry real impression volume but are IT-outsourcing buyer
intent, not Pulumi's audience.

* Fix: shorten meta_desc to satisfy 160-char lint rule

* Address pre-merge review: drop unverifiable citation, fix style flags

- Drop the Puppet 43% figure's citation to a press release that
  doesn't carry the statistic; keep the qualitative point instead.
- Use Pulumi Policies instead of CrossGuard in the tools table
  (STYLE-GUIDE.md).
- Soften the platformengineering.org 2026 predictions claim from
  "as its top trend" to "among its predictions" since ranking
  wasn't confirmed.
- Remove the duplicate Team Topologies paragraph under "How to get
  started"; cross-reference the fuller team-building section instead.
- Style: drop wordy "It is", difficulty-qualifier "just", weasel word
  "usually", and directional above/below references per
  STYLE-GUIDE.md Inclusive Language section.

* Address pre-merge review: fix subject-verb agreement, apply style suggestions

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Co-authored-by: workprentice <257153108+workprentice@users.noreply.github.com>

Author: workprentice[bot]