Add support for custom sidebars

The working groups section of the Eclipse website has a custom sidebar layout:

• https://www.eclipse.org/org/workinggroups/adoptium-charter.php
• https://www.eclipse.org/org/workinggroups/process.php

From my testing, there is no easy way to get subsections in Hugo. Every page in /org/working-groups/ will apply the /org section sidebar.

My proposed solution is to a front matter param that allows us to target specific menu identifiers. This solution will also add support for multiple menu sections, allowing us to recreate the layout shown above.

Here is a preview of what I have implemented so far: https://preview-238--www-eclipse-org.eclipsecontent.org/org/working-groups/adoptium/charter/

I will need to update solstice assets to add proper styling for parent items that do not have a URL, as well as update some of the padding.