[codex] docs: add releases link to installation

[abhijeetnardele24-hash]

Summary

• add a direct GitHub Releases link near the top of the installation page
• make the standalone binary download path easier to discover for users who prefer direct binaries

Closes #751.

Validation

• reviewed the rendered markdown diff locally
• npx prettier --check docs/installation.md could not run because this repo pins devEngines.runtime.version to Node 22 and this machine is running Node 24

[bolt-new-by-stackblitz]

Run & review this pull request in StackBlitz Codeflow.

[vercel]

@abhijeetnardele24-hash is attempting to deploy a commit to the pnpm Team on Vercel.

A member of the Team first needs to authorize it.