RFR: JDK-8176509 Use pandoc for converting build readme to html

[Magnus Ihse Bursie]

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Please note that the *content* of the README file is severly outdated 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.

Bug: https://bugs.openjdk.java.net/browse/JDK-8176509

There's no easy way to get WebRev to both incorporate cross-repo files 
*and* show diffs for moved files.

So here's one that shows all files:
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8176509-use-pandoc-for-build-readme/webrev.01

And here's one that shows the diff for the moved files:
http://cr.openjdk.java.net/~ihse/JDK-8176509-use-pandoc-for-build-readme/webrev.03

/Magnus