doc: Update Equations doc with dvisvgm and dvipng references

Fixes #619.

doc/ox-hugo-manual.org

@@ -1453,11 +1453,15 @@ of these is true:
 :EXPORT_HUGO_CUSTOM_FRONT_MATTER: :mathjax true
 :END:
 #+begin_description
-Supporting {{{latex}}} equations
+{{{latex}}} equations support
 #+end_description
 By default, the inline and block equations are exported to Markdown in
 a format that can be rendered using [[https://www.mathjax.org/#gettingstarted][MathJax]]. /You can find one MathJax
 config example [[#mathjax-config-example][below]]./
+
+~ox-hugo~ indirectly extends from ~ox-html~ and so it also inherits a
+different way of exporting {{{latex}}} equations --- by [[#exporting-equations-to-images][exporting them
+to images]].
 **** Inline equations
 - Inline equations are wrapped between =\(= and =\)=.
   - =$= wrapping also works, but it is not preferred as it comes with
@@ -1477,29 +1481,26 @@ For example, below in Org:
 LaTeX formatted equation: \( E = -J \sum_{i=1}^N s_i s_{i+1} \)
 #+end_src
 
-will look like this in Hugo rendered HTML:
+will look like this in Hugo rendered HTML (using MathJax):
 
 LaTeX formatted equation: \( E = -J \sum_{i=1}^N s_i s_{i+1 }\)
 
-*Don't see this in Markdown, see what it looks after Hugo has
-processed it.* --- =ox-hugo= does some heavy escaping to get around a
-Blackfriday issue with supporting MathJax syntax equations (that Org
-supports).
-
-Here's another example, taken from [[https://orgmode.org/manual/LaTeX-fragments.html][(org) LaTeX fragments]].
+Here's another example, taken from [[info:org#LaTeX fragments]]:
 
-Below in Org:
 #+begin_example
 If $a^2=b$ and \( b=2 \), then the solution must be either
 $$ a=+\sqrt{2} $$ or \[ a=-\sqrt{2} \]
 #+end_example
 
-renders to:
+Above renders to below using Mathjax:
 
 If $a^2=b$ and \( b=2 \), then the solution must be either
 $$ a=+\sqrt{2} $$ or \[ a=-\sqrt{2} \]
 
-(Note that the last two equations show up on their own lines.)
+#+begin_note
+Note that the last two equations show up on their own lines because
+those equations are wrapped in ~\[ .. \]~.
+#+end_note
 **** {{{latex}}} Environments
 =ox-hugo= support {{{latex}}} environments.
 
@@ -1512,25 +1513,50 @@ C = W\log_{2} (1+\mathrm{SNR})
 \end{equation}
 #+end_src
 
-will export as:
+will render as below using MathJax:
 
 \begin{equation}
 \label{eq:1}
 C = W\log_{2} (1+\mathrm{SNR})
 \end{equation}
 
-Equation referencing will also work[fn:3]. So =\ref{eq:1}= will render as
-\ref{eq:1}.
-**** Examples
-You can find many examples by looking at {{{testtag(equations)}}}.
+#+begin_note
+You can find many more equation examples at {{{testtag(equations)}}}.
+#+end_note
+***** Referencing Equations
+Equation referencing will also work **but it requires MathJax**. [[#mathjax-config-example][Here]]
+is a MathJax configuration that is known to work for this referencing
+feature. =\label{SOMELABEL}= labeled equations can be referenced using
+=\ref{SOMELABEL}=[fn:12]. So =\ref{eq:1}= will render as \ref{eq:1}
+and link to the equation above.
 **** MathJax config example
 :PROPERTIES:
 :CUSTOM_ID: mathjax-config-example
 :END:
 Here's how MathJax is configured for this website:
 
-1. hugo-onyx-theme [[https://github.com/kaushalmodi/hugo-onyx-theme/blob/master/layouts/partials/mathjax.html][~mathjax.html~ partial]]:
-2. MathJax config ( [[https://github.com/kaushalmodi/hugo-onyx-theme/blob/master/static/js/mathjax-config.js][~js/mathjax-config.js~]] ):
+- hugo-onyx-theme [[https://github.com/kaushalmodi/hugo-onyx-theme/blob/master/layouts/partials/mathjax.html][~mathjax.html~ partial]]
+  - MathJax config: [[https://github.com/kaushalmodi/hugo-onyx-theme/blob/master/static/js/mathjax-config.js][~js/mathjax-config.js~]]
+**** Exporting {{{latex}}} equations as images
+:PROPERTIES:
+:CUSTOM_ID: exporting-equations-to-images
+:END:
+If you prefer, you can using ~dvisvgm~ or ~dvipng~[fn:3] to export the
+equations to SVG or PNG images, and the equations in the Org file will
+be replaced by links to those images in the exported Markdown files.
+
+The method for exporting the equations to images is the same as that
+documented for ~ox-html~ in [[info:org#Math formatting in HTML export]]:
+- To export the equations to SVG, add ~#+options: tex:dvisvgm~ to the
+  top of your Org file, or add ~:EXPORT_OPTIONS: tex:dvisvgm~ to the
+  post subtree's property drawer.
+- Similarly, to export the equations to PNG, use the ~tex:dvipng~
+  option instead.
+
+#+begin_note
+Referencing to equations using ~\ref{..}~ will not work if equations
+are exported as images.
+#+end_note
 *** Tags and Categories
 :PROPERTIES:
 :EXPORT_FILE_NAME: tags-and-categories
@@ -4661,6 +4687,12 @@ Categories,)}}}.
 * Footnotes
 
 
+
+
+[fn:3] The ~dvisvgm~ and ~dvipng~ executables ship with TexLive
+distributions.
+
+[fn:12] This same MathJax setup is used on the =ox-hugo= test site too.
 [fn:11] If you are looking for ~EXPORT_HUGO_SECTION*~ property, that
 has been deprecated; please see {{{titleref(Deprecation
 Notices#export-hugo-section-frag)}}}.
@@ -4696,11 +4728,6 @@ the default ~DONE~. And similarly, use your custom "TODO" keyword
 instead of the default ~TODO~ (ensure that it is *not* in
 ~org-done-keywords~).
 
-[fn:3] This requires MathJax. [[https://github.com/kaushalmodi/hugo-bare-min-theme/blob/master/layouts/partials/mathjax.html][Here]] is a MathJax configuration that is
-tested to work for equation referencing (referencing =\label{}=
-labeled equations using =\ref{}=). The same is used on the =ox-hugo=
-test site too.
-
 [fn:2] For only subtree-based exports, you can set that special tag as
 Org style tags too. Example: ~* I don't want to export this post
 :no_no_dont_export:~.. and don't forget to add that tag to