Introduce "Sensible Folder Structure"

This aims to setup initial infrastructure to keep state files
inside `crafter-config-var-directory`.

- A setting is created to enable/disable the behaviour (enabled by default)
- A helper function is introduced to ease adding new state files
- This is all documented

Unrelated to the above is:

We add `CUSTOM_ID` to a documentation entry to enable creating an internal link.

This is because according to [orgmode documentation](https://orgmode.org/manual/Internal-Links.html) local
links by ID work only when `CUSTOM_ID` is set.

> (...) a construct like ‘[[#my-custom-id]]’  specifically targets the
entry with the ‘CUSTOM_ID’ property set to ‘my-custom-id’.

Attempting to export `README.org` without this results in an error.

README.org

@@ -68,6 +68,17 @@ external packages, for example:
   functionality)
 - *Possibly* =vc-mode= by default
 
+** Sensible folder layout
+
+While Emacs tends to keep everything (code, configuration, state files, ...)
+inside `user-emacs-directory` modern computer systems tend to keep those
+separated.
+
+Crafted Emacs tries to maintain some balance between those two paradigms by
+bringing just the right amount of order to it.
+
+See [[file:docs/crafted-emacs.org#folder-structure][Folder structure]] in the documentation for more details.
+
 ** Works well in the terminal
 
 Some people prefer to use Emacs in the terminal instead of as a graphical
@@ -146,6 +157,9 @@ information about how they can be configured!
 - crafted-shell :: A starter configuration for =eshell= and =vterm=
 
 * Customization
+:PROPERTIES:
+:CUSTOM_ID: customization
+:END:
 
 To add your own customization to this configuration, create a configuraton file
 in one of the following directories:

docs/crafted-defaults.org

@@ -231,3 +231,35 @@
     #+begin_src emacs-lisp
       (savehist-mode -1)
     #+end_src
+
+  - =crafted-folders= : =t=
+
+    Enables the use of a modern folder structure, as explained below.
+
+    You can turn this off by adding this code to =config.el= (or
+    through the Customization UI):
+
+    #+begin_src emacs-lisp
+      (customize-set-variable 'crafted-folders nil)
+    #+end_src
+
+** Sensible path defaults
+
+Loading this module will make your Emacs, by default, save
+several runtime and/or configuration files inside Rational Emacs
+=var= directory as explained in [[file:crafted-emacs.org#folder-structure][the documentation]].
+
+This behaviour can be disabled by customizing =crafted-folders=
+as explained above.
+
+The list of paths being updated is growing as we aim to have
+all state, runtime and/or configuration files stored following
+modern practices. What is currently altered is:
+
+- [[help:bookmark-default-file][bookmark-default-file]]
+- [[help:nsm-settings-file][nsm-settings-file]]
+- [[help:org-id-locations-file][org-id-locations-file]]
+- [[help:project-list-file][project-list-file]]
+- [[help:savehist-file][savehist-file]]
+- [[help:save-place-file][save-place-file]]
+- [[help:tramp-persistency-file-name][tramp-persistency-file-name]]