refactor!: restructure Doom core

BREAKING CHANGE: This restructures the project in preparation for Doom
to be split into two repos. Users that have reconfigured Doom's CLI
stand a good chance of seeing breakage, especially if they've referred
to any core-* feature, e.g.

  (after! core-cli-ci ...)

To fix it, simply s/core-/doom-/, i.e.

  (after! doom-cli-ci ...)

What this commit specifically changes is:
- Renames all core features from core-* to doom-*
- Moves core/core-* -> lisp/doom-*
- Moves core/autoloads/* -> lisp/lib/*
- Moves core/templates -> templates/

Ref: #4273

templates/QUICKSTART_INTRO

@@ -0,0 +1,19 @@
+But before you doom yourself, here are some things you should know:
+
+1. Don't forget to run 'doom sync', then restart Emacs, after modifying
+   ~/.doom.d/init.el or ~/.doom.d/packages.el.
+
+   This command ensures needed packages are installed, orphaned packages are
+   removed, and your autoloads/cache files are up to date. When in doubt, run
+   'doom sync'!
+
+2. If something goes wrong, run `doom doctor`. It diagnoses common issues with
+   your environment and setup, and may offer clues about what is wrong.
+
+3. Use 'doom upgrade' to update Doom. Doing it any other way will require
+   additional steps. Run 'doom help upgrade' to understand those extra steps.
+
+4. Access Doom's documentation from within Emacs via 'SPC h d h' or 'C-h d h'
+   (or 'M-x doom/help')
+
+Have fun!