Add Wiki support for AsciiDoc include directive

What does this MR do?

This MR fixes an error when the Wiki attempted to render the AsciiDoc include directive.

An AsciiDoc include directive allows people to embed the content of one file within another (see https://asciidoctor.org/docs/user-manual/#include-directive).

GitLab added support for the AsciiDoc include directive in 3f5d7c7e.

The Wiki #show view was missing the expected variables needed for Gitlab::Asciidoc::IncludeProcessor#resolve_target_path to confirm that the file existed in the repository before allowing its content to be included.

An AsciiDoc include directive would previously cause a 500 error to be raised.

#195625 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers