Add metadata chapter

Signed-off-by: trivernis <trivernis@protonmail.com>

chapters/metadata.md

@@ -0,0 +1,45 @@
+# Metadata / Element Attributes
+
+Metadata or element attributes are a way to change the behaviour of some
+elements.
+
+The syntax for metadata is the same syntax used for defining keys of bibliography entries
+in the document.
+
+```
+String value
+[key = value]
+
+String value
+[key = "String value"]
+
+Integer value
+[key = 123]
+
+Float value
+[key = 1.23]
+
+Boolean
+[key = false]
+[key] equals [key=true]
+```
+
+There are some elements that support metadata:
+
+```
+Hide a section (including subsections) in the TOC
+#[toc-hidden] Section
+
+Set the size of an image
+!(url)[width = 42%, height=auto, brightness=10, contrast=1.2, huerotate=180, invert, grayscale]
+
+Set the source of a quote
+[author=Me date=[[date]] display="{{author}} - {{date}}"]> It's me
+
+Set options for placeholders
+[[toc]][ordered]
+
+Set the type of an import
+<[style.css][type=stylesheet]
+```
+&nbsp;

main.md

@@ -57,3 +57,6 @@
 
 <[chapters/placeholders.md]
 
+- - -
+
+<[chapters/metadata.md]