docs(software-packs): add catalog and stub pages

[khuyentran1401]

Reference Issues or PRs

Fixes #662. Part of #631.

What does this implement/fix?

• Adds a Software Packs catalog landing page with an installation overview and a single table listing every available pack.
• Adds 11 per-pack stub pages (Chat, Data Science, LGTM, LLM Serving, Longhorn Backup, MLflow, Nebi, Pi Coding Agent, Provenance Collector, Ray Serve, Superset), each linking to its upstream repository for canonical install steps.
• Defines what a community pack is and routes contributors to the Build your own pack flow.

Testing

• Did you test the pull request locally?
• Did you add new tests?

Verified the catalog page, all 11 stubs, sidebar entries, and internal links build cleanly with npx docusaurus build.

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