Also rename a few things. `typedoc` module is now `reference` and `mkdocs` module is now `site`.
Moved a couple things into `util` as well
The `nav` module will be in stacked PR
- Better grouping of `validate` command options in `--help`
- Move some more constants into the module
- Removed the "guess" functions and replaced them with functions which use `which` to actually find the necessary executables
- Moved `isStringArray()` to `util`
- Fixed some error messages and added more
- Simplified use of `DocutilsValidator#fail()`
- Removed option for custom path to `requirements.txt`
This modifies the validator to collect all of the errors from validation (except those that are "unexpected" though not _so_ unexpected that I did not account for them; stuff like "unparseable output from `pip list --json`") and display them as they happen without aborting the entire validation process.
