docs: convert to portable Markdown and wire docs.docker.com validation

[dgageot]

This implements the docker-agent-side work for Phase 1 and Phase 2.2 of #3371, which enables the docs to be single-sourced across github.io and docs.docker.com via a Hugo module mount. The two sites have different rendering pipelines, and the existing Liquid-template links, callout divs, and permalink declarations are incompatible with Hugo, so they need to be replaced with constructs that both engines understand natively.

The first commit converts 91 content files to portable Markdown. All 292 {{ '…' | relative_url }} links are replaced with plain relative links (resolved by jekyll-relative-links on github.io, natively by Hugo on docs.docker.com). The 186 Liquid callout <div> blocks become GitHub-style > [!NOTE]/> [!TIP]/> [!WARNING] alerts that Hugo understands out of the box. Card and feature-grid HTML on content pages is converted to Markdown lists; the landing page keeps its HTML but is now Liquid-free. Redundant permalink: front matter is dropped, keywords: entries are added, and a small number of broken anchors and root-absolute links are fixed. STYLE.md and the markdownlint config are updated for the new conventions. Jekyll rendered a byte-identical 92-page URL set before and after this change.

The second commit makes the GitHub-style alerts render correctly on the existing Jekyll site. Kramdown emits > [!NOTE] as a plain blockquote rather than a styled panel, so a small upgradeAlerts() function in app.js upgrades the rendered blockquote to the existing callout panel style client-side, without re-parsing innerHTML and matching only direct-child <p> markers to avoid false positives.

The third commit adds CI coverage: a docs.yml workflow runs markdownlint and an offline lychee link check (with fragment validation) on every docs change, and a docs-upstream.yml workflow calls docker/docs' reusable validate-upstream workflow (SHA-pinned) so the mount configuration can be validated automatically once Phase 2.1 lands in docker/docs.

Part of #3371.

[aheritier]

👋 This PR has merge conflicts with the base branch. Please rebase or merge the latest base branch and resolve them. I've moved it to draft and added status/needs-rebase; it'll be picked back up automatically once the conflicts are cleared.

[docker-agent]

Assessment: 🟢 APPROVE

Reviewed 97 changed files (docs content conversions, CI workflows, JS alert-upgrade function, markdownlint config). No bugs or correctness issues were introduced by this PR.

The Liquid-to-portable-Markdown conversion, > [!NOTE/TIP/WARNING] alert syntax, CI workflow configuration, and the upgradeAlerts() JavaScript function all look correct. The relative-link rewrites, front-matter changes, and lychee/markdownlint CI additions are clean.