[Docs] Add basic gcluster job submit guide to README and incorporate PR #5492 edits

[SwarnaBharathiMantena]

Summary

This PR updates README.md to introduce high-level customer-facing documentation for gcluster job submit and incorporates technical writing review edits from PR #5492.

Key Changes

1. gcluster job submit Overview & Quickstart:
   • Added a minimal ## Job Submission (gcluster job submit) section to README.md.
   • Documented prerequisites (GCLUSTER_IMAGE_REPO, kubectl, GKE auth plugin).
   • Added example usage for pre-built images and on-the-fly image builds (--build-context).
   • Included persistent CLI configuration setting examples (gcluster job config set).
   • Linked to the detailed Gcluster Job Submission Guide for advanced topics (storage mounts, Kueue priority queues, multi-slice topologies).
2. Technical Writing Cleanups (PR Restructure and edit information on README.md #5492):
   • Streamlined product overview and main component breakdown.
   • Updated pre-built bundle execution platform compatibility matrix and download steps.
   • Formatted all raw text URLs into clean, relative Markdown links.
   • Preserved .tgz tarball bundle release steps from upstream/develop.

Testing & Validation

• Verified Markdown formatting and relative documentation links.
• Confirmed alignment with existing CLI flags and technical documentation guidelines.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the project's primary documentation by introducing a dedicated section for the 'gcluster job submit' command and incorporating various technical writing refinements. These changes aim to provide users with a clearer, more accessible quickstart experience while ensuring the documentation remains aligned with current best practices.

Highlights

• Documentation Update: Added a comprehensive guide for 'gcluster job submit' to the README, covering prerequisites, pre-built image usage, on-the-fly image building, and configuration defaults.
• Technical Writing Improvements: Streamlined the product overview, updated the bundle compatibility matrix, and converted raw text URLs into clean, relative Markdown links to improve readability and maintenance.

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[gemini-code-assist]

Code Review

This pull request significantly refactors and updates the README.md file to improve clarity, streamline installation instructions, and document the new gcluster job submit command. Feedback is provided to correct a grammatical issue where the noun "setup" was used instead of the verb phrase "set up".

[SwarnaBharathiMantena]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates the README.md to simplify the project description and introduces a comprehensive section on the gcluster job submit command, detailing prerequisites, job submission examples, and job management. The review feedback suggests clarifying that GCLUSTER_IMAGE_REPO requires the full Artifact Registry repository path to prevent push failures, and recommends replacing an absolute GitHub URL with a relative link for internal repository references.