Rename HTML File in "docs/" Folder

[tajmone]

In order for the build HTML book to be viewable on GitHub Pages it should be named index.hml. This requires some changes in the build toolchain.

• Change docs_src/build.sh so that:
  • it builds the HTML doc in ../hugo-book.html (root folder).
  • Copies of ../hugo-book.html to docs/index.html.

The above will ensure that the root folder of the repo will contain both the AsciiDoc preprocessed version of the book as well as its HTML version properly named:

• hugo-book.asciidoc
• hugo-book.html

This is what most users will expect, so it justifies having a redundant copy of the HTML book in docs/, because the file named index.html is only useful for the website.