[Docs] Updating sidebar to add Xdebug pages

[fellyph]

This pull request updates the sidebar to add links to the Xdebug and contributor guides documentation. This Pull request also includes renaming documentation titles for better readability.

Documentation improvements:

• Renamed the Xdebug introduction documentation page and its title to "Debugging with Xdebug in WordPress Playground" for increased clarity and alignment with user expectations.
• Updated the contributor day table lead guide title and heading to "How to Lead a Contributor Day Table" and "Contributor Day Table Leadership Guide" to make the guide's purpose more explicit.

Sidebar organization:

• Added the new contributor day table lead guide (main/contributing/contributor-day-table-lead) to the contributing section of the sidebar for easier access.
• Created a new "Xdebug on Playground" sidebar category, including links to the introduction and getting started guides, making Xdebug documentation easier to find and navigate.