- Rename regexp.rdoc to exclude from "Pages".  This file is for to be
  included in the "class Regexp" document, but it also appeared as a
  separate page duplicately.
- Fix links on case-sensitive filesystems.
- Fix to use rdoc-ref instead of converted HTML page names.

Before:
```
* + corresponds to $+, which contains last capture group.
```

After:
```
* \+ corresponds to $+, which contains last capture group.
```

* Add missing space for `String#start_with?`.
* Add missing pluses for `String#tr` and
  `Methods for Converting to New String` label.
* Move quote into the tag for `Whitespace in Strings` label.