docs(onprem): make the self-hosting page a real landing page

[devGregA]

installation_options.md was two sentences and a sales address. That was fine when it was one of three pages in the section; it isn't now that it sits above pages covering sizing, installation, migration, upgrades, backups, upload limits, and storage.

It now does what a landing page should:

• Explains the two deployment models and what separates them. A single Compose host has to be sized for peak rather than average, because nothing in that arrangement scales out and the peak is usually a large import landing while people are working in the UI. Kubernetes runs the same components as separate workloads, so you provision for steady state and add replicas when load arrives. The page gives a recommendation instead of leaving the reader to work it out, and it says a single host is a legitimate choice rather than a compromise.
• States the database position: external managed for production, which is what the Helm chart assumes, with the containerized option called out as suitable for evaluation and not for production data.
• Covers what you need before starting, including that sizing depends on both finding count and concurrent users because those drive different parts of the deployment, and how to obtain a license and the tooling.
• Keeps both contact paths. The original page sent everyone to the sales address. Existing customers now get their account representative or support, and evaluators still get sales. Sending a prospect to a support address would have been a regression.

Notes for review

Frontmatter is deliberately untouched. The diff hunks start at line 8, so this does not collide with the weight: 4 to weight: 5 change in #15459. The two PRs can merge in either order.

No cross-links yet. Most of the pages this one describes are still in open PRs, so linking them would fail the offline link check on this base. The section navigation covers discovery in the meantime, and the links belong in a single pass once #15452, #15455, #15456, #15458, #15459, and #15461 have landed.

Testing

• hugo --minify --gc --config config/production/hugo.toml builds clean with no new warnings.
• No dead internal links. Both email links render through the theme's obfuscation hook with noscript fallbacks, verified in the built output.

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