docs: rebrand main for 26.08 RC GitHub Pages QA review

[kheiss-uwzoo]

Summary

• Rebrand reader-facing version strings on main from 26.05 to 26.08 / 26.8.0 so GitHub Pages matches what QA will review during 26.08 RC readiness analysis.
• Update release notes heading, install RC example (26.08-RC1), Helm README/OpenShift image tag examples, and cross-doc anchor links for the recommended minimal install section.
• Add explicit callouts that docs.nvidia.com remains on the frozen 26.05 / 26.5.0 publish snapshot until a separate post-GA publish run.

Out of scope (intentionally unchanged)

• docs/publish/versions.json — version picker for docs.nvidia.com
• .github/workflows/nrl-docs-nvidia-publish.yml — still checks out 26.05 / defaults to 26.5.0
• nemo_retriever/helm/Chart.yaml, values.yaml — runtime/chart defaults (eng PR when RC artifacts exist)

Merge timing

Hold as draft until 26.08 RCs start Monday; merge to main when QA should begin reviewing GitHub Pages under the 26.08 label.

Test plan

• After merge, confirm GitHub Pages deploy shows 26.08 in release notes and Helm README examples
• Confirm docs.nvidia.com version picker still lists 26.5.0 as latest until publish workflow is run
• Spot-check anchor links to #recommended-minimal-install-2608 from deployment-options and prerequisites-support-matrix