Set AsciiDoc attributes related to the requested path

What does this MR do?

Asciidoctor promises to automatically perform some attribute substitutions: https://asciidoctor.org/docs/user-manual/#env-attributes Hardly any of these substitutions are performed when GitLab renders an Asciidoc document. Those attributes that are related to the path of the file can be implemented easily, since the request path is known when we invoke Asciidoctor. This MR implements support for those attributes.

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

This is a minor patch with an improvement outside the critical infrastructure.

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team