move releases.html to 4.00.1.html

This will make it easier to manage new releases. We'll be able to leave
4.00.1.html as-is, create a new file for the new release, and change
the symbolic link latest.html to the new file.
3.12.1.html: fix title
3.12.1.html: add link to earlier releases
4.00.1.html: fix title
4.00.1.html: move alternative compilers info
move What's New content into respective release pages

This makes more sense now that we have a page for each major release.
It is also consistent with how it was on caml.inria.fr.