Convert gendocs.sh to use rake yard

Instead of sdoc, let's just use YARD docs which we promote as a documentation standard anyway.

## Verification

From the top of the metasploit-framework checkout:

  - [ ] `documentation/gendocs.sh`
  - [ ] See that documentation is generated in `doc/`
  - [ ] Take a break while it runs. A few errors will be thrown but nothing disasterous.
  - [ ] `xdg-open doc/index.html` (or firefox or safari or whatever)