Backport PR matplotlib#27096: make fonts.py, mathtext.py, text_intro.…

…py confirm to docs guidelines

galleries/users_explain/text/fonts.py

@@ -73,7 +73,7 @@
   - Limited support with Matplotlib
 
 Font subsetting
-~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^
 
 The PDF and PostScript formats support embedding fonts in files, allowing the
 display program to correctly render the text, independent of what fonts are
@@ -93,7 +93,7 @@
 Currently Type 3, Type 42, and TrueType fonts are subsetted.  Type 1 fonts are not.
 
 Core Fonts
-~~~~~~~~~~
+^^^^^^^^^^
 
 In addition to the ability to embed fonts, as part of the `PostScript
 <https://en.wikipedia.org/wiki/PostScript_fonts#Core_Font_Set>`_ and `PDF

galleries/users_explain/text/mathtext.py

@@ -283,7 +283,7 @@
 # (not minus).
 #
 # Custom fonts
-# ~~~~~~~~~~~~
+# ^^^^^^^^^^^^
 # Mathtext also provides a way to use custom fonts for math.  This method is
 # fairly tricky to use, and should be considered an experimental feature for
 # patient users only.  By setting :rc:`mathtext.fontset` to ``custom``,

galleries/users_explain/text/text_intro.py

@@ -238,7 +238,7 @@
 # locations, and how they are labelled.
 #
 # Terminology
-# ~~~~~~~~~~~
+# ^^^^^^^^^^^
 #
 # *Axes* have an `matplotlib.axis.Axis` object for the ``ax.xaxis`` and
 # ``ax.yaxis`` that contain the information about how the labels in the axis
@@ -254,7 +254,7 @@
 # that format the tick labels.
 #
 # Simple ticks
-# ~~~~~~~~~~~~
+# ^^^^^^^^^^^^
 #
 # It is often convenient to simply define the
 # tick values, and sometimes the tick labels, overriding the default
@@ -287,7 +287,7 @@
 
 # %%
 # Tick Locators and Formatters
-# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 #
 # Instead of making a list of all the ticklabels, we could have
 # used `matplotlib.ticker.StrMethodFormatter` (new-style ``str.format()``
@@ -380,7 +380,7 @@ def formatoddticks(x, pos):
 
 # %%
 # Dateticks
-# ~~~~~~~~~
+# ^^^^^^^^^
 #
 # Matplotlib can accept `datetime.datetime` and `numpy.datetime64`
 # objects as plotting arguments.  Dates and times require special