docs(azure): document buying Enterprise through Azure Marketplace

[sandroqdb]

Summary

QuestDB Enterprise is now listed on Azure Marketplace. The docs had an AWS Marketplace section but nothing for Azure. This PR documents the Azure route and makes clear what it is: a way to buy QuestDB Enterprise, not a hosted service or a one-click deployment.

Changes

documentation/deployment/azure.md: new "Buy through Azure Marketplace" subsection under the existing "Enterprise on Azure" section.

• Links to the public listing: https://marketplace.microsoft.com/en-us/product/questdb_com.c5f84985-0214-403e-bdd3-5d436252a35b
• States the benefit: QuestDB Enterprise goes on the customer's existing Azure invoice and through their existing Azure procurement process.
• States the deployment model: the listing is a purchasing route. QuestDB Enterprise still runs in the customer's own Azure subscription on Virtual Machines, Docker or Kubernetes, and the infrastructure guidance on the page applies.
• Five steps mirroring the listing's own "How to buy on Azure" text: contact sales, receive a private offer, accept and subscribe in the marketplace, receive the Enterprise binaries or container images, follow the Enterprise Quick Start.

documentation/getting-started/enterprise-quick-start.md: one bullet under the licence requirement pointing Azure buyers to the new section.

Why it sits under "Enterprise on Azure"

The AWS page puts its marketplace section under "Deployment" because the AWS listing is a pre-configured AMI you launch. The Azure listing is different: it is a transactable offer for the Enterprise subscription, and the customer then deploys the binaries or containers themselves. Placing it in the Enterprise section avoids implying a one-click Azure deployment that does not exist.

Deliberately left out

• No claim that purchases count towards a Microsoft Azure Consumption Commitment (MACC). The listing does not show the "Azure benefit eligible" badge, so this is unconfirmed. Easy to add later if Microsoft confirms eligibility.

Verification

Deploy preview (https://preview-560--questdb-documentation.netlify.app/docs/deployment/azure/#buy-through-azure-marketplace):

• The new section renders under "Enterprise on Azure" as an H3 and appears in the right-hand table of contents.
• Both paragraphs and all five steps render as written. The listing link, the sales@questdb.com mailto and the Enterprise Quick Start link are all present and correct.
• On the Enterprise Quick Start page, the new "Buying through Azure?" bullet renders under the licence requirement. Clicking it navigates to /docs/deployment/azure/#buy-through-azure-marketplace and lands with the new heading in view.

CI: "PR preview" (full production build), "Validate broken links" and gitleaks all pass. This covers the build I could not complete locally (local node_modules has an unrelated ajv version mismatch).

Content: every claim in the new section is taken from the published listing text.

Notes

A companion PR in questdb/questdb.io (#3028) switches the homepage Azure Marketplace button from "Coming soon" to a live link. The two are independent and can merge in any order.

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