Add multiline LaTeX equation example to wiki

- Ref: https://codeberg.org/forgejo/forgejo/issues/6902

docs/user/wiki.md

@@ -126,3 +126,19 @@ $ \frac{1}{2} = 0.5 $
 This will then result in the following properly rendered equation:
 
 ![The $\frac{1}{2} = 0.5$ equation properly rendered by KaTeX](../_images/user/wiki/katex_example.png)
+
+It is also possible to have LaTeX render in display mode, using, `$$` or `\[` with `\]`:
+
+```markdown
+\[ \mathbf{R} = \frac 1M \iiint_Q \rho(\mathbf{r}) \mathbf{r} \mathrm dV \]
+$$ (p \implies q) \iff (\neg q \implies \neg p) $$
+```
+
+It is however not possible to use this when the equation spans multiple lines, in that case codeblock with the language `math` should be used, for example:
+
+````markdown
+```math
+\int_0^\pi\ln(1-2\alpha\cos x+\alpha^2) \mathrm dx =
+2\pi\ln|\alpha|
+```
+````