feature: add support for GLFM math blocks

This add supports for GLFM's math blocks [1] using hugo's Markdown
render hooks [2].

[1]: https://docs.gitlab.com/ee/user/markdown.html#math
[2]: https://gohugo.io/templates/render-hooks/#render-hooks-for-code-blocks

Signed-off-by: Stephan Lachnit <stephanlachnit@debian.org>

layouts/_default/_markup/render-codeblock-math.html

@@ -0,0 +1,3 @@
+<p>$$
+{{ .Inner | safeHTML }}
+$$</p>

userguide/content/en/docs/adding-content/diagrams-and-formulae/index.md

@@ -35,6 +35,13 @@ $$\tag*{(1)} P(E) = {n \choose k} p^k (1-p)^{n-k}$$
 The probability of getting \\(k\\) heads when flipping \\(n\\) coins is:
 $$\tag*{(1)}  P(E) = {n \choose k} p^k (1-p)^{n-k}$$
 
+You can also use [GLFM's math blocks](https://docs.gitlab.com/ee/user/markdown.html#math) (requires hugo 0.93 or newer):
+````markdown
+```math
+\tag*{(1)} P(E) = {n \choose k} p^k (1-p)^{n-k}
+```
+````
+
 {{% alert title="Tip" %}}
 This [wiki page](https://en.wikibooks.org/wiki/LaTeX/Mathematics) provides in-depth information about typesetting mathematical formulae using the \\(\LaTeX\\) typesetting system.
 {{% /alert %}}