docs/doc-guide: Add documentation on SPHINX_IMGMATH

Now that building html docs with math expressions does not need texlive
packages, remove the note on the requirement in the "Sphinx Install"
section.

Instead, add sections of "Math Expressions in HTML" and "Choice of Math
Renderer".
Describe the effect of setting SPHINX_IMGMATH in the latter section.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/a67e3279-6bc7-ee2c-2b49-9275252460b0@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: akiyks

Documentation/doc-guide/sphinx.rst

@@ -48,10 +48,6 @@ or ``virtualenv``, depending on how your distribution packaged Python 3.
       on the Sphinx version, it should be installed separately,
       with ``pip install sphinx_rtd_theme``.
 
-   #) Some ReST pages contain math expressions. Due to the way Sphinx works,
-      those expressions are written using LaTeX notation. It needs texlive
-      installed with amsfonts and amsmath in order to evaluate them.
-
 In summary, if you want to install Sphinx version 2.4.4, you should do::
 
        $ virtualenv sphinx_2.4.4
@@ -86,6 +82,27 @@ Depending on the distribution, you may also need to install a series of
 ``texlive`` packages that provide the minimal set of functionalities
 required for ``XeLaTeX`` to work.
 
+Math Expressions in HTML
+------------------------
+
+Some ReST pages contain math expressions. Due to the way Sphinx works,
+those expressions are written using LaTeX notation.
+There are two options for Sphinx to render math expressions in html output.
+One is an extension called `imgmath`_ which converts math expressions into
+images and embeds them in html pages.
+The other is an extension called `mathjax`_ which delegates math rendering
+to JavaScript capable web browsers.
+The former was the only option for pre-6.1 kernel documentation and it
+requires quite a few texlive packages including amsfonts and amsmath among
+others.
+
+Since kernel release 6.1, html pages with math expressions can be built
+without installing any texlive packages. See `Choice of Math Renderer`_ for
+further info.
+
+.. _imgmath: https://www.sphinx-doc.org/en/master/usage/extensions/math.html#module-sphinx.ext.imgmath
+.. _mathjax: https://www.sphinx-doc.org/en/master/usage/extensions/math.html#module-sphinx.ext.mathjax
+
 .. _sphinx-pre-install:
 
 Checking for Sphinx dependencies
@@ -164,6 +181,38 @@ To remove the generated documentation, run ``make cleandocs``.
 	  as well would improve the quality of images embedded in PDF
 	  documents, especially for kernel releases 5.18 and later.
 
+Choice of Math Renderer
+-----------------------
+
+Since kernel release 6.1, mathjax works as a fallback math renderer for
+html output.\ [#sph1_8]_
+
+Math renderer is chosen depending on available commands as shown below:
+
+.. table:: Math Renderer Choices for HTML
+
+    ============= ================= ============
+    Math renderer Required commands Image format
+    ============= ================= ============
+    imgmath       latex, dvipng     PNG (raster)
+    mathjax
+    ============= ================= ============
+
+The choice can be overridden by setting an environment variable
+``SPHINX_IMGMATH`` as shown below:
+
+.. table:: Effect of Setting ``SPHINX_IMGMATH``
+
+    ====================== ========
+    Setting                Renderer
+    ====================== ========
+    ``SPHINX_IMGMATH=yes`` imgmath
+    ``SPHINX_IMGMATH=no``  mathjax
+    ====================== ========
+
+.. [#sph1_8] Fallback of math renderer requires Sphinx >=1.8.
+
+
 Writing Documentation
 =====================