- Distinguish between rustdoc and rustc
- Use `rustdoc` instead of `doc` subcommand

This only profiles the last crate and also avoids special casing
RUSTDOCFLAGS.

- Add a fake rustdoc
- Create symlink automatically if it doesn't already exist

- Add 'Doc' to CLI help
- Make Doc a default BuildKind when --builds is not passed

- Don't panic if looking at two local builds in the dashboard
- Don't panic if there are no pstat results available

- Change `subcommand()` to take BuildKind and return Option
- Remove `is_build_kind_supported`

This ensures that the doc build will succeed when run later, since
rustdoc and rustc are known to have different ICEs.

This avoids rustdoc-fake getting out of date when rustc-fake is updated.