Documentation is difficult to maintain

[alpaca-tc]

There are a number of HTML documents under git control under docs/.
Regenerating these shows that they have numerous diffs and are not well maintained.

cd docs
make build-docs
git diff HEAD | wc -l
    4653

In my opinion, there is no need to keep HTML documents under git control.
I think it would be better to keep only the .rst files under git control and build the generated HTML during the release process.

I am not familiar with the release process for this gem, so I am not sure if this opinion is nice.