Add support for sidebar, example and openblock in AsciiDoc

What does this MR do and why?

Contrbutes to #362325 (closed)

Problem

1. CSS classes for special AsciiDoc blocks are sanitized from the output
2. CSS classes are missing definitions

Solution

• Add sidebarblock, openblock and exampleblock to the allowlist.
• Add CSS styles for new blocks.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How it looks on https://asciidoc.org/ (scroll down to the online editor)

How to set up and validate locally

1. Create a snippet with the content from $3679077
2. You should see that blocks sidebar and example have CSS styles