What's new page fed by the latest release's changelog highlights (replaces #2401)

[jh-RLI]

Summary

Closes #2382. Replaces #2401 (tomi-rli, credited as co-author), rebuilt on the new homepage from #2352.

Placement (maintainer's call): a link plus its own page. Bryan's homepage sections are unchanged.

• /whats-new/ (base:whats-new) shows the latest release's highlights on the static-page layout. Page header: About, breadcrumb Home / What's new, where "Home" links to the homepage.
• The homepage hero shows "See what's new →" under its two buttons, only when there are highlights. The homepage footer's Platform column also links "What's new".

No page names the release version (maintainer's call). Production's version should not be easy to read off the platform. There is no version heading, the hero link names no version, and there is no full-changelog link, because its URL carried the version. latest_release_highlights() returns only HTML, so no template can print a version, and a test holds both pages free of the VERSION string. This is obscurity only: the highlights text matches a public changelog on GitHub.

Source: a ## Highlights section in each changelog. It holds three to five points written for users at release time; nothing is extracted from the developer changelog.

• base.helper.latest_release_highlights() takes the newest released changelog (x_y_z.md, never current.md) that has highlights, picked by version number and cached per process.
• A patch release without highlights keeps the previous release's highlights on the page. Feature 2382 changelog highlights #2401 broke on exactly this: it took the newest file, which today is 1_10_1.md, a patch release with no highlights, so the homepage would have shown an empty "Latest release" box.
• It replaces read_version_changes, whose version/changes no template has read since the homepage redesign.
• current.template.md and current.md gain the section. current.md carries a draft for the next release (new homepage and consistent look, the tables tab, the access drawer, datasets as drafts). Please reword it as you see fit.
• RELEASE_PROCEDURE.md gains the step. The new template is annotated in REUSE.toml.

Dropped from #2401:

• its index.html changes (the old layout, with broken markup)
• the unused render_markdown tag
• dataedit/static/release_info_startpage/release_v1.8.0.md
• the edit to the released 1_8_0.md
• the mkdocs page

Nothing shows until v1.11.0 is released, because only released changelogs count. To preview, copy current.md to 1_11_0.md and restart the server.

Tests

• base/tests/test_whats_new.py: newest release by number, only the section, patch release skipped when empty or comment-only, current.md ignored, the page with and without highlights, the hero link with and without them, no version on either page.
• base:whats-new joins the page lint and the view smoke test. base green.
• Checked in headless Chrome at 1440 and 390 px. A local dev server shows the hero link invisible (blue on blue) until manage.py compress runs, because index-style.css sits in an offline compress block. A deploy runs compress.

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