Add how to add footnotes in a markdown file. (#718)

It could be useful to have this information in the documentation.

Co-authored-by: bgg <bgg@bombe>
Reviewed-on: https://codeberg.org/Codeberg/Documentation/pulls/718
Reviewed-by: Bastian Greshake Tzovaras <gedankenstuecke@noreply.codeberg.org>
Reviewed-by: Robert Wolff <mahlzahn@posteo.de>
Co-authored-by: bgg <bgg@noreply.codeberg.org>
Co-committed-by: bgg <bgg@noreply.codeberg.org>

content/markdown/introduction-to-markdown.md

@@ -45,12 +45,12 @@ This gets rendered as
 This is **bold text**.
 
 ```markdown
-This is also **bold text**.
+This is also __bold text__.
 ```
 
 This gets rendered as
 
-This is also **bold text**.
+This is also __bold text__.
 
 ### Italics
 
@@ -62,12 +62,12 @@ to get the same effect.
 Here are a few examples.
 
 ```markdown
-This is _italic text_.
+This is *italic text*.
 ```
 
 This gets rendered as
 
-This is _italic text_.
+This is *italic text*.
 
 ```markdown
 This is also _italic text_.
@@ -77,6 +77,32 @@ This gets rendered as
 
 This is also _italic text_.
 
+### Strikethrough
+
+To strikethrough use `~~` around the text.
+
+```markdown
+This is ~~strikethrough text~~.
+```
+
+is rendered as
+
+This is ~~strikethrough text~~.
+
+### Footnotes
+
+To add footnotes use the syntax `[^name]` inline, and define them with:<br>
+`[^name]:text.` 
+
+```markdown
+Text with a footnote.[^1]
+[^1]: A footnote.
+```
+is rendered as 
+
+![Footnote Example rendering](/images/markdown/Footnote-example.png)
+
+
 ## Forgejo-specific formatting
 
 ### Emoticons
@@ -128,3 +154,4 @@ E.g.
 is rendered to:
 
 ![Mermaid Example rendering](/images/markdown/mermaid-example.png)
+