Fix link on 404 documentation page

[jonirouAtTalan]

Current behavior

The following links on 404 documentation page are broken :

Exemple page : https://docs.typo3.org/Home/ExtensionManuals.html#extension-manuals

Or pick one of the following:

[Core Documentation](https://docs.typo3.org/typo3cms/References/Index.html)
[Third-party extensions](https://docs.typo3.org/typo3cms/extensions/Index.html)
[System extensions](https://docs.typo3.org/typo3cms/SystemExtensions/Index.html)

Expected behavior/output

These links should target sections on this page : https://docs.typo3.org/m/typo3/docs-how-to-document/main/en-us/Basics/BasicPrinciples.html

Possible Solution

Update the link