Rename CSS units as CSS types.

docs/css_types/index.md

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+# CSS Types
+
+CSS types define the values that Textual CSS rules accept.
+
+CSS types will be linked from within the [styles reference](../styles/index.md) in the "Formal Syntax" section of each rule.
+The CSS types will be denoted by a keyword enclosed by angle brackets `<` and `>`.
+
+For example, the style [`align-horizontal`](../styles/align.md) references the CSS type [<horizontal>](./horizontal.md):
+
+--8<-- "docs/snippets/syntax_block_start.md"
+align-horizontal: <a href="./horizontal.md">&lt;horizontal&gt;</a>
+--8<-- "docs/snippets/syntax_block_end.md"

docs/styles/css_units/index.md

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-# CSS units
-
-Many CSS rules accept units of some sort, as opposed to a set of values.
-
-The different CSS units are:
-
- - [color](./color.md) – e.g., to set the background color of a widget;
- - [fractional](./fractional.md) – e.g., to set the opacity of a widget;
- - [integer](./integer.md) – e.g., to set the size of a grid layout;
- - [percentage](./percentage.md) – e.g., to set the transparency of colors;
- - [scalar](./scalar.md) – e.g., to set the dimensions of a widget; and
- - [text style](./text_style.md) – e.g., to style the text of a widget.