Update and edit Configuring Permissions in Sys Admin Guide

- Write new intro
- Describe policy files and policy dir
- Note that you can log yourself out of Identity settings if you're not careful
- Correct the sawtooth identity command name (it's not identity-tp)
- Delete future-design paragraph at end
- Add links to relevant info
- Number procedure steps
- Fix formatting throughout (including changing indents)

Also:

- Correct the first "sawset proposal create" command to start with sudo,
  include the --key option

  Also: use fancy $(cat ...) syntax to provide the user's actual public key
  and add a note that this command MUST be run on the "genesis node"

- Replace the second "sawset proposal create" command to refer to
  the first one (the procedure assumes it has already been done)

Signed-off-by: Anne Chenette <chenette@bitwise.io>