docs: align README with AI quickstart standards

[itay1551]

Summary

Align the root README (and related docs) with rh-ai-quickstart CONTRIBUTING README structure requirements for AI quickstarts.

Requirements we addressed

• Short description ≤160 characters — trimmed the intro under the H1 (title kept as-is by choice; action-verb rule still not met).
• Architecture diagrams — renamed H3 and improved image alt text.
• Minimum hardware requirements — specific GPU / storage / CPU guidance (not a one-line table cell).
• Minimum software requirements — OpenShift 4.20+, OpenShift AI 3.4 (3.3 minimum), path-specific tools for Existing / ROSA / ARO.
• Required user permissions — cloud admin + cluster-admin for ROSA/ARO; cluster-admin for existing OpenShift.
• Delete — undeploy make commands for existing OpenShift; links to ARO and ROSA teardown sections.
• Tags — Title, Description, approved Industry: Media and IT services, Product, Use case (removed non-standard Business challenge).

What we changed

• Restructured README.md to match the quickstart section order (requirements H3s, validating, delete, tags).
• Added PCA_Deployment_ROSA/README.md Destroying the Cluster (terraform destroy) so Delete can link like ARO.
• Updated docs/requirements.md to match OpenShift 4.20+ / RHOAI 3.4 (min 3.3) and simplified permissions wording.

Test plan

• Confirm short description is ≤160 characters
• Confirm required headings exist: Architecture diagrams, Minimum hardware/software, Required user permissions, Delete, Tags with Industry
• Confirm Delete links resolve for Existing make targets, ARO #destroying-the-cluster, ROSA #destroying-the-cluster
• Spot-check ToC links in root README and ROSA README

[Hadar301]

Great improvement for the docs!

[manujoy7]

this is a good structure. lets keep ARO and ROSA based deployment artifacts and docs separately.
In the readme doc, lets make open code as the primary harness and others as optional.