📝 docs(platforms): fix RST formatting and TOC hierarchy (#447)

The platforms documentation had RST formatting issues that affected the
table of contents structure. 📌 A leftover "Default paths" header in the
preamble with no corresponding section content has been removed to clean
up the introduction.

Platform-specific sections (macOS, Windows, Linux/Unix, Android) were
using dashes (`---`) which render as level-2 headers, incorrectly
appearing as main sections in the TOC. ✨ These are now level-3 headers
(using `~~~`) to align them logically with "User directories" and
"Shared directories" subsections they precede.

The result is a cleaner, more intuitive table of contents that better
reflects the actual documentation structure.

Author: gaborbernat

docs/platforms.rst

@@ -5,8 +5,6 @@ Platform details
 This page describes the default paths for each platform and any platform-specific behavior.
 
 All examples below assume ``appname="SuperApp"`` and ``appauthor="Acme"`` unless stated otherwise.
-Default paths
--------------
 
 User directories
 ~~~~~~~~~~~~~~~~
@@ -419,7 +417,7 @@ See also: :ref:`api:Shared binary directory`
    `Chocolatey <https://docs.chocolatey.org/en-us/choco/setup>`_.
 
 macOS
------
+~~~~~
 
 On macOS, ``platformdirs`` uses the standard Apple ``~/Library`` directories by default.
 See `Apple's File System Programming Guide
@@ -439,7 +437,7 @@ the Homebrew prefix as an additional path when ``multipath=True``.
    :no-index:
 
 Windows
--------
+~~~~~~~
 
 On Windows, ``platformdirs`` uses the Shell Folder APIs to resolve directories.
 See `Microsoft's Known Folder documentation
@@ -534,7 +532,7 @@ Empty or whitespace-only values are ignored and the normal resolution applies.
    :no-index:
 
 Linux / Unix
-------------
+~~~~~~~~~~~~
 
 On Linux and other Unix-like systems, ``platformdirs`` follows the
 `XDG Base Directory Specification <https://specifications.freedesktop.org/basedir/latest/>`_.
@@ -571,7 +569,7 @@ the corresponding ``XDG_*_DIRS`` variable, joined by ``:``.
    :no-index:
 
 Android
--------
+~~~~~~~
 
 On Android, ``platformdirs`` uses the app's private storage directories. The app's package
 folder (e.g. ``/data/data/com.example.app``) is detected via ``python-for-android`` or