Greptile Summary

This PR backports GA release documentation updates from main (#2213) to the 26.05 branch, replacing all RC build tag references (26.05-RC1) with the GA version string 26.5.0 and updating the repo-level README to reflect the stable release.

• nemo_retriever/README.md: All three install pin examples updated to nemo-retriever==26.5.0 (local GPU, remote NIM, nemotron-parse).
• nemo_retriever/helm/README.md: Air-gap mirroring section updated to reference 26.5.0 instead of 26.05-RC1.
• README.md: Branch description now says "GA PyPI and Helm chart version 26.5.0" instead of the RC tag wording.
• docs/docs/extraction/releasenotes.md: Added a GA PyPI install bullet with a quickstart link targeting the 26.05 branch.

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[26.05 branch docs] --> B{Version string}
    B -->|Before| C["26.05-RC1"]
    B -->|After| D["26.5.0 (GA)"]
    D --> E[README.md]
    D --> F[nemo_retriever/README.md\n3 install examples]
    D --> G[nemo_retriever/helm/README.md\nAir-gap section]
    D --> H[docs/docs/extraction/releasenotes.md\nGA PyPI bullet added]

_{Reviews (2): Last reviewed commit: "docs(nrl): revert optional Helm chart pi..." | Re-trigger Greptile}