Render bracket references as superscript text

This commit improves markdown rendering to convert reference labels
(e.g., `[1]`) to superscripts, improving document readability without
cluttering the text. This improvement applies documentation of all
scripts and categories.

Changes:

- Implement superscript conversion for reference labels within markdown
  content, ensuring a cleaner presentation of textual references.
- Enable HTML content within markdown, necessary for inserting `<sup>`
  elements due to limitations in `markdown-it`, see
  markdown-it/markdown-it#999 for details.
- Refactor markdown rendering process for improved testability and
  adherence to the Single Responsibility Principle.
- Create `_typography.scss` with font size definitions, facilitating
  better control over text presentation.
- Adjust external URL indicator icon sizing for consistency, aligning
  images with the top of the text to maintain a uniform appearence.
- Use normal font-size explicitly for documentation text to ensure
  consistency.
- Remove text size specification in `markdown-styles` mixin, using `1em`
  for spacing to simplify styling.
- Rename font sizing variables for clarity, distinguishing between
  absolute and relative units.
- Change `font-size-relative-smaller` to be `80%`, browser default for
  `font-size: smaller;` CSS style and use it with `<sup>` elements.
- Improve the logic for converting plain URLs to hyperlinks, removing
  trailing whitespace for cleaner link generation.
- Fix plain URL to hyperlink (autolinking) logic removing trailing
  whitespace from the original markdown content. This was revealed by
  tests after separating its logic.
- Increase test coverage with more tests.
- Add types for `markdown-it` through `@types/markdown-it` package for
  better editor support and maintainability.
- Simplify implementation of adding custom anchor attributes in
  `markdown-it` using latest documentation.

src/presentation/assets/styles/_mixins.scss

@@ -1,5 +1,6 @@
 @use "@/presentation/assets/styles/colors" as *;
 @use "@/presentation/assets/styles/fonts" as *;
+@use "@/presentation/assets/styles/typography" as *;
 
 @mixin hover-or-touch($selector-suffix: '', $selector-prefix: '&') {
   @media (hover: hover) {
@@ -69,6 +70,11 @@
   list-style: none;
 }
 
+@mixin reset-sup {
+  vertical-align: unset;
+  font-size: unset;
+}
+
 @mixin reset-button {
   margin: 0;
   padding-block: 0;
@@ -93,7 +99,7 @@
 
 @mixin flat-button($disabled: false) {
   @include reset-button;
-  $font-size: $font-size-normal;
+  $font-size: $font-size-absolute-normal;
 
   @if $disabled {
     color: $color-primary-light;