RFR: 8378366: Accessibility issues in CSS Reference Guide and Introduction to FXML

[Andy Goryachev]

Addressing the accessibility issues in JavaFX CSS Reference Guide and Introduction to FXML documents:

[JDK-8378306](https://bugs.openjdk.org/browse/JDK-8378306) In JavaFX CSS Reference Guide, a `<ul>` element must appear within an `<li>` element
[JDK-8378308](https://bugs.openjdk.org/browse/JDK-8378308) In JavaFX CSS Reference Guide, <textarea> elements do not contain a label
[JDK-8378309](https://bugs.openjdk.org/browse/JDK-8378309) In JavaFX CSS Reference Guide and Introduction to FXML, there are color contrast issues
[JDK-8378310](https://bugs.openjdk.org/browse/JDK-8378310) In JavaFX CSS Reference Guide and Introduction to FXML, a "Skip to Main Content" link doesn't exist
[JDK-8378307](https://bugs.openjdk.org/browse/JDK-8378307) In JavaFX CSS Reference Guide, some tables don't use the scope attribute or the <caption> element

For ease of reviewing, the first 5 commits correspond to individual tickets.

Most of the changes are trivial, but for [JDK-8378307](https://bugs.openjdk.org/browse/JDK-8378307) I decided to replace the table with `<ul>` list used elsewhere in similar situations.

In [JDK-8378308](https://bugs.openjdk.org/browse/JDK-8378308) the `<textarea>`s were made read-only because obviously.

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Commit messages:
 - 8378307 tables
 - 8378310 skip to main content
 - 8378309 contrast
 - 8378308 textarea label
 - 8378306 li

Changes: https://git.openjdk.org/jfx/pull/2082/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2082&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378366
  Stats: 118 lines in 3 files changed: 17 ins; 50 del; 51 mod
  Patch: https://git.openjdk.org/jfx/pull/2082.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2082/head:pull/2082

PR: https://git.openjdk.org/jfx/pull/2082