rework spack help (#3033)

- Full help is now only generated lazily, when needed.
  - Executing specific commands doesn't require loading all of them.
  - All commands are only loaded if we need them for help.

- There is now short and long help:
  - short help (spack help) shows only basic spack options
  - long help (spack help -a) shows all spack options
  - Both divide help on commands into high-level sections

- Commands now specify attributes from which help is auto-generated:
  - description: used in help to describe the command.
  - section: help section
  - level: short or long

- Clean up command descriptions

- Add a `spack docs` command to open full documentation
  in the browser.

- move `spack doc` command to `spack pydoc` for clarity

- Add a `spack --spec` command to show documentation on 
  the spec syntax.