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

modules/lang/emacs-lisp/demos.org

@@ -7,7 +7,7 @@ usage in the documentation displayed by either ~describe-function~ or
 ~helpful-function~ (on [[kbd:][SPC h f]]).
 
 * Table of Contents :TOC_3:
-- [[#core-lib][core-lib]]
+- [[#doom-lib][doom-lib]]
   - [[#add-hook][add-hook!]]
   - [[#add-transient-hook][add-transient-hook!]]
   - [[#after][after!]]
@@ -38,7 +38,7 @@ usage in the documentation displayed by either ~describe-function~ or
   - [[#unsetq-hook][unsetq-hook!]]
   - [[#use-package][use-package!]]
 
-* core-lib
+* doom-lib
 ** add-hook!
 :PROPERTIES:
 :added: pre-3.0.0