RFR: 8313810: BoxLayout uses <blockquote> instead of list for layout options

[Alexey Ivanov]

A clean-up of documentation for [`BoxLayout`](https://docs.oracle.com/en/java/javase/17/docs/api/java.desktop/javax/swing/BoxLayout.html#class-description):

1. Use the `<ul>` element instead of `<blockquote>` to list layout options or axes.
2. Add links to `GridBagLayout` and `Box` classes.
3. Markup references to classes, mostly `BoxLayout` itself, with `{@code}`.
4. Separate the introduction to layout description from the description of horizontal layout. They're now in different paragraphs for easier scanning.
5. Add the missing ‘a’ articles.
6. Organise imports, including expanding the wildcard import.

The updated version: [**`BoxLayout.html`**](https://cr.openjdk.org/~aivanov/8313810/api/java.desktop/javax/swing/BoxLayout.html#class-description) (without the CSS).

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Commit messages:
 - Add links to GridBagLayout and Box classes
 - Optimize imports
 - 8313811: Improve description of how BoxLayout lays out components
 - 8313810: BoxLayout uses <blockquote> instead of list for layout options

Changes: https://git.openjdk.org/jdk/pull/15182/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15182&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313810
  Stats: 34 lines in 1 file changed: 14 ins; 1 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/15182.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15182/head:pull/15182

PR: https://git.openjdk.org/jdk/pull/15182