Correct the custom wiki sidebar instructions

What does this MR do?

This MR clarifies the instructions to create a custom wiki sidebar. The correct way to add customizations to the sidebar is not to create a file _sidebar, but to create a page _sidebar which then results in a file _sidebar.md created in the wiki repository.

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done