Since JDK-8139668, the build README html file has been generated from a markdown source.

Unfortunately, this implementation used the original markdown (from https://daringfireball.net/projects/markdown/), which was quite limited, and did not for instance allow for tables.

Now that JEP 299 is about to standardize on using pandoc to convert markdown to man pages, I think it would be reasonable to use it to convert the build README to html as well.

I also used this opportunity to clean up redundant and/or incorrect information spread in the READMEs all over the forest, and to move the build READMEs to a better location.

Please note that the *content* of the README file is severly outdated (not to mention unstructured and sometimes irrelevant), but updating that is not part of the scope for this bug. I will follow up on it, though.