- IRB has its own documentation in the project readme and https://ruby.github.io/irb/ already.
- As a tool, its internal implementation details are not relevant to the user.
- Excluding it from the documentation generation reduces the size of the generated documentation
and makes search results more relevant.
I also checked the there are no references to IRB so it should not break any links.
RDoc options that do not change and can be written in `.rdoc_options`
file are moved, so that they match when called without `make`.
Get rid of parsing the files in `page_dir` twice (as relative paths
and absolute paths).
