[openstreetmap/openstreetmap-website] Update 404 page (Issue #3532)

[Berrely]

### Description

### The problem
The current 404 page that users get when typing in an invalid URL is uninformative, clunky and ugly.
* Instead of hierarchically displaying the most important information, it opens the prompt with “File not found”, only mentioning the specific error at the end of the second line.
* It has a lot of redundancy (“OpenStreetMap server”), and doesn't give good advice on what to do.
* The second line says that if you have found a broken link or bug, you should ask the OSM community, even though the correct forums for bugs would be repositories like this one.
* It also looks incredibly ugly, with loads of whitespace and the default browser font.
### The solution
* The 404 page should be redesigned(!)
* * The text should be rewritten to be more helpful
* * The header should be more clear on the specific error
* * Links should be provided to get back to the homepage (currently hidden behind the giant logo)
* * The bug text should be removed or refactored to be more appropriate

See also a possible mockup attached.

### Screenshots

**Current 404 page:**
![](https://i.imgur.com/Bk5b1lB.png)
**Mockup of possible 404 page:**
![image](https://user-images.githubusercontent.com/20139168/163435807-5a2602e0-1adb-4fd2-946d-90980c3ad49d.png)


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