Update the stale documentation in the source tree.

The OpenZGY documentation is generated by Doxygen, so the latest version can be found in the build output. How to download this is a bit tricky. So for those that just want a quick preview there is a version checked in to the source tree. This needs to be updated ever so often.

After we got the gitlab CI/CD set up it is somewhat easier to access the latest version since you at least don't need to download and build the entire project. So I am removing 6 of the 8 versions, leaving only the PDF version of the public API. I have also added a suitable warning in the documentation main page and in a separate README.md.

At some point the documentation built on gitlab might be deployed to a proper web server that allows the html version to be displayed online. If and when that happens the last two pdfs can be removed.

I will need at least two commits. The first one to rebuild the documentation including the new warning and the second one to commit the pdf files.