scaladoc should warn if date in file name of blogpost and in its content differ

## Compiler version

`3.0.0-RC1`

## Minimized code

See https://github.com/lampepfl/dotty/commit/f4b053694cdf96d79f743232ee1851d0a29ca1d0#diff-3f13b71dd80ca93a540bed8e3eb982fece4180d0c4d80b68bf3b544a1c432328


## Expectation

Either only one date should be allowed or scaladoc should  report error/warning in case dates are not equal
