feat: publish new JS course

[honzajavorek]

The aim of this PR is to publish the new JS course as described in the PR description of #1584, and to unlist the old JS course. The old one should be still accessible for a grace period.

Replacing the old JS course with a new one, which is identical to the Python course, has been previously sanctioned by both Ondra and Michał.

The Plan

• The scraping_basics_javascript root leads to the new JS course.
• The pages of the old JS course move to legacy/web-scraping-for-beginners. It's gonna be a read-only archive. Must be noindex to avoid cannibalization issues.
• The web-scraping-for-beginners, i.e. the root of the old JS course URLs, leads to redirects which take people to corresponding pages in the new JS course. This lets us use the SEO juice from the old URLs.
• The redirects add #old-js-course to the URL. The new JS course pages contain a component which, if #old-js-course is present in the URL, displays a commemorative plaque about the change and link the old JS course. This improves UX: "Hey, you have until 1.1.2026 to go through this course. After that please refer to the newly updated JS course ."
• At some point in future, we'll nuke the archive of the old JS course and link Internet Archive instead in the commemorative plaque.

The Plan is a result of a long discussion between Michał, Aleš, and me, which takes into account both the UX of existing users of the JS course and SEO.

Related Work

• Depends on refactor: introduce a shared folder for partials and images from both JS and Python courses #1889
• Closes feat: use the Python course as a template for a new JavaScript course #1584
• Closes feat: re-structure the JS course so that it's a flat list of numbered lessons #1579
• Fixes Update Academy's approach to setting up a Node.js project #947
• Discovered Verify that both the new beginner courses explain what to do if the website is client-side rendered #1900
• Closes feat: add conditional admonition component #2009 (PoC)
• Contains style: few fixes to make linters happy #2023
• Closes Update the Web scraping basics for JavaScript devs course #1550

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