Introduce CSS to Asciidoctor sources

[MBoegers]

In this Issue the prior work of PR #13 and issue #16 should be combined.

Description of the issue

In PR #13 @hendrikebbers provides a basic styling for the Adoptium documentation.
These classes have to be applied to all adoc files in the repository.

In issue #16 @hendrikebbers and @MBoegers aggreed on:

• linking the CSS files at build/render-time
• stick with Ascidoctors default behavior
  For an example, see section specify the custom stylesheet.

Acceptance

• configure build.sh to use the stylesheets
• configure GitHub Action build to use the stylesheets

Questions

If you have any questions, just ask us directly in this issue by adding a comment. You can join our community chat at Slack. Next to this you can find a general manual about open-source contributions here.

[hendrikebbers]

Hey @MBoegers will you join the meeting today? #35 Next to this what is the best way to contact you?

[hendrikebbers]

linking adoptium/documentation-services#44

[MBoegers]

@hendrikebbers pling ;)