docs(integrations): add new Integrations section (Phase 1)

[gavinelder]

Summary

Phase 1 of the Platform Cloud Integrations migration. Stands up a new top-level Integrations section under `platform-cloud/docs/` to consolidate cloud provider permissions, credentials, git providers, container registries, data sources, and identity into one discoverable home — and starts deprecating the older fragmented locations.

This PR is content-additive only: nothing is removed in this phase. Existing URLs (`/platform-cloud/credentials/`, `/platform-cloud/git/`, IAM sections inside `compute-envs/*`) continue to resolve. The old Credentials sidebar entry is preserved as Credentials (legacy) until Phase 3.

Biggest user-messaging win

The AWS IAM/permission content previously scattered across:

• `compute-envs/aws-batch.md` — 83 IAM mentions
• `compute-envs/eks.md` — 71 mentions
• `compute-envs/aws-cloud.md` — 18 mentions

…is now consolidated into one tabbed reference at `integrations/cloud-providers/aws/iam-policies.md` (tabs: AWS Batch / AWS Cloud / Amazon EKS).

What's included

• 39 new files under `platform-cloud/docs/integrations/`
  • Cloud providers (16): AWS, Azure, GCP — overview, IAM/role policies, credentials, data/storage access, manual setup
  • Git providers (9): split out from `git/overview.md` into per-provider pages, plus separate Seqera AI Git access page
  • Container registries (9): reparented from `credentials/`
  • Data sources, identity, agent (4): duplicated for Phase 1
  • Overviews (3): Integrations hub, Cloud providers hub
• Updated `cloud-sidebar.json`: new `Integrations` category, old Credentials retained as `Credentials (legacy)`

Migration phases

• ✅ Phase 1 — Stand up new section (this PR)
• ⏳ Phase 2 — Cross-link with deprecation banners, update internal callers (next stacked PR)
• ⏳ Phase 3 — Strip IAM content from compute-envs, delete legacy paths, add redirects
• ⏳ Phase 4 — Editorial review pass on `integrations/`

Full plan in `/Users/gavin.elder/.claude/plans/hey-i-want-to-hidden-yao.md`.

Open questions for review

1. Confirm Integrations as the section name (vs. Connections, Credentials & integrations).
2. For Phase 3 redirects: add `@docusaurus/plugin-client-redirects` (recommended) or use Netlify-level?
3. Confirm scope stays cloud-only — Enterprise IA divergence is accepted for some period.
4. Should EKS Fusion v2 IRSA + `aws-auth` content live under `integrations/cloud-providers/aws/eks-additions` (current placement) or stay under `compute-envs/eks.md`?
5. Studios, Secrets, Resource Labels are arguably integrations too — confirm scope is "credentials/IAM/git only" for now.

Test plan

• `docusaurus build` passes with no broken links or anchors
• Reviewer: spot-check the AWS / Azure / Google overview pages, confirm setup checklists make sense
• Reviewer: spot-check sidebar — both new `Integrations` and `Credentials (legacy)` should appear
• Reviewer: confirm tabbed AWS `iam-policies.md` renders correctly in preview
• Stakeholder review: marketing/SE confirm the Integrations section name

🤖 Generated with Claude Code

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[justinegeffen]

Thanks, @gavinelder! These look great. There's a lot of new content in this series of PRs. Would you be able to provide some context around the changes/additions? We'd like to ensure we prioritize it all appropriately. Thank you!

[gavinelder]

So there is actually nothing new here it's pulling out existing nested comment into its own section and it was more for appropriateness