Fixes #13983

Two changes have been made:
- In AkkaBuild, forced using UTF-8 to verify HTML files containing
  diagrams
- In the documentation chapter Building Akka, added a section clarifying
  the Graphviz dependency of generating Scaladoc Content Hierarchies

These fixes eliminates the issues I faced while building Akka on Windows
with NetBeans. The first issue is a Windows specific issue where the
default charset cannot be UTF-8. The second is Windows related in the
sense that Graphviz, the optional dependency of Scaladoc, had to be
manually resolved.
Updates in the Building Akka documentation page in
accordance with the discussion with @bantonsson
Merge pull request #15011 from csomogyi/master

Fixing minor issues I found while building Akka on Windows