make link to API docs more prominent in README

[jmtd]

          > @jmtd We should add a reference to this doc in the README.md file so that it's easily accessible. What do you think?

It's there -- two sentences above "How to build the images". Perhaps it should be more prominent...

Originally posted by @jmtd in https://github.com/jboss-container-images/openjdk/issues/74#issuecomment-1699363790