docs: remove outdated documentation

[julienrbrt]

Description

ref: #13456

I came across outdated documentation again and broken links. This removes them.

We do not need to merge this now (although we must before v0.47 tagging).
We still need to go over all the documentation again, so more will probably come.

If the rendering issue is too pressing: https://docs.cosmos.network/main/core/baseapp#checktx, then we should merge, I'll create another PR later for removing outdated documentation.

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