refactor: simplify courses

[elbotho]

It's a bit annoying because courses had so much refactoring already…
But I gave this more thought and did more SEO research I think I'm pretty happy with this approach.

The idea is to put all course page below each other on one page with some space in between the pages.
Navigation between the pages then works by hash-links (or if users prefer by scrolling).

Demo

This can be reviewed and discussed, but not merged yet (needs cf-worker redirects).

Thoughts:

• UX: We want to keep the "step by step" feeling to not overwhelm the learners with content (done with vertical space)
• UX: We want quick navigation between the steps (is immediate now)
• SEO: Many course pages are pretty short an that's not great for SEO. So all on one page is probably better overall
• SEO: While Google says they value invisible, but accessible content (like in accordions) the same as visible content test show they really clearly prefer visible content (no content is hidden)
• DX: Having one url per entity is definitely cleaner and technically easier with less special treatment for courses
• DX: one less custom serlo variable in the editor: always nice

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Name	Status	Preview	Updated (UTC)
frontend	✅ Ready (Inspect)	Visit Preview	Apr 1, 2025 1:04pm

[github-actions]

📦 Next.js Bundle Analysis for @serlo/frontend

This analysis was generated by the Next.js Bundle Analysis action. 🤖

⚠️ Global Bundle Size Increased

Page	Size (compressed)
global	105.56 KB (🟡 +3 B)

Details

The global bundle is the javascript bundle that loads alongside every page. It is in its own category because its impact is much higher - an increase to its size means that every page on your website loads slower, and a decrease means every page loads faster.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

If you want further insight into what is behind the changes, give @next/bundle-analyzer a try!

Twenty Pages Changed Size

The following pages changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/[...slug]	107.17 KB (🟢 -17 B)	212.73 KB
/___bot_or_not	114.85 KB (🟢 -17 B)	220.41 KB
/___editor_preview	202.13 KB (-16 B)	307.69 KB
/___old_comments	97.26 KB (-1 B)	202.82 KB
/biologie	175.56 KB (-17 B)	281.12 KB
/chemie	175.56 KB (-17 B)	281.12 KB
/content-only/[...slug]	96.07 KB (🟢 -17 B)	201.63 KB
/discussions	93.8 KB (-1 B)	199.36 KB
/entity/create/[type]/[taxonomyId]	223.64 KB (🟢 -73 B)	329.2 KB
/entity/repository/add-revision/[...id]	223.31 KB (🟢 -73 B)	328.87 KB
/entity/repository/compare/[entity_id]/[revision_id]	111.41 KB (🟢 -17 B)	216.97 KB
/informatik	175.56 KB (-17 B)	281.12 KB
/lerntipps	175.56 KB (-17 B)	281.12 KB
/license/detail/[id]	82.83 KB (🟢 -17 B)	188.39 KB
/mathe	175.56 KB (-17 B)	281.12 KB
/nachhaltigkeit	175.56 KB (-17 B)	281.12 KB
/page/create	223.31 KB (🟢 -73 B)	328.87 KB
/taxonomy/term/create/[parent_id]/[id]	223.02 KB (🟢 -73 B)	328.58 KB
/user/profile/[username]	194.25 KB (-17 B)	299.81 KB
/user/settings	225.04 KB (🟢 -73 B)	330.59 KB

Details

Only the gzipped size is provided here based on an expert tip.

First Load is the size of the global bundle plus the bundle for the individual page. If a user were to show up to your website and land on a given page, the first load size represents the amount of javascript that user would need to download. If next/link is used, subsequent page loads would only need to download that page's bundle (the number in the "Size" column), since the global bundle has already been downloaded.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

Next to the size is how much the size has increased or decreased compared with the base branch of this PR. If this percentage has increased by 20% or more, there will be a red status indicator applied, indicating that special attention should be given to this.

[hejtful]

Appreciate the SEO tests!