Migrate web-sk and web-en to new topic pages

[jankrcal]

The design of new topic pages fits the best to web-cz which already has a lot of content.

Nevertheless, technically, it would be beneficial to migrate all the sites to the new topic pages. It can be simpler than for web-cz:

• Design-wise, we can keep the accordion on index.md to showcase most of the content we have, just adjust the links.
• We can keep the intro texts shorter and don't have any intro texts and Q&As for subsections.
• We can translate the existing dashboards and "In the nutshell" but we also don't need to use them to cut down on maintenance costs.

Clean-up list:

• jekyll-archives plugin
• tag.html
• tags.yaml and tags list in the content and tags param for tags.html include (unless we repurpose them for search)
• check whether weights for regular content are used anywhere (at least on web-cz)
• tag based implementation of various pieces of UI that got forked by [Topic pages] Publish topic pages #183 (navigation.html, related.html, tags.html)
• any css specific to tag.html