doc: add the list to be checked .gemspec before release
doc: avoid duplicated replacing from _index.html to alphabetical_index.html

We should replace filenames/contents of ./doc/referene/**/*.html only,
because "reference:publication:prepare" task doesn't always replace
all html files and no copied files in doc/html/ directory are replaced
again. We also don't have to do all html files in doc/ directory.