Doom profiles documentation: run doom env --profile

### I confirm that...

- [X] I have searched the [issue tracker](https://github.com/doomemacs/doomemacs/issues), [documentation](https://docs.doomemacs.org), [FAQ](https://docs.doomemacs.org/-/faq), [Discourse](https://discourse.doomemacs.org), and [Google](https://google.com), in case this issue has already been reported/resolved.
- [X] I have read "[How to Debug Issues](https://doomemacs.org/d/how2debug)", and will use it to provide as much information about this issue as possible.
- [X] The issue can be reproduced on the **latest** available commit of Doom.
- [X] The issue can be reproduced on a stable release of Emacs, such as 27, 28, or 29. *(Unstable versions end in .50, .60, or .9x)*

### Expected behavior

After going through the documentation I expected that profiles work.

### Current behavior

Weird things happen, e.g., magit complains that there is no emacsclient.

### Steps to reproduce

Follow: https://github.com/doomemacs/doomemacs/tree/master/profiles/

Create a profile in `~/.config/doom-profiles.el`:
```lisp
;; -*- mode: emacs-lisp; -*-
((default (user-emacs-directory . "~/.emacs.d")
(doom-user-dir . "~/.config/doom/default/"))))
```

To solve this, simply run: `doom env --profile default`.
This should be documented (if it is the correct thing to do).

### System Information

https://pastebin.com/KLdY39pR