docs: remove schema based validation from readme

[braaar]

Description

According to the roadmap #178 and several comments by @NoNameProvided, schema based validation is on its way out. Additionally, it is currently broken. Due to the limited maintainer resources on this project, it is probably unwise to spend effort on trying to fix this feature before removing it again in the next major version.

Therefore I have made this PR removing the schema based validation instructions from the readme in order to avoid further confusion.

We could remove the actual code for this feature, since it doesn't work. But I suppose that is technically a breaking change, so I'm not sure what's the best way to go about that

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified)
• documentation added or updated
• I have run the project locally and verified that there are no errors

Fixes

fixes #595, fixes #1155

[NoNameProvided]

Landed in 7cd7546. Thanks!

(I have to cherry-pick due to merge conflict.)

[github-actions]

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