New collapsible panels in settings made it harder to find what we're looking for

We've recently made all settings to be expandable in collapsible panels:

Issues

This made it looking cleaner and better, but we have now 2 problems:

1. People are struggling to find the settings they want, now they have to expand each panel to find what they are looking for.
2. We made all the docs that mention any settings out-of-date.

Examples::

• Sid took some time to find "remove forking relationship"
• A user couldn't find repo mirroring settings (https://gitlab.com/gitlab-org/gitlab-ee/issues/4302)
• A user couldn't find 2FA settings (https://gitlab.com/gitlab-org/gitlab-ce/issues/41028)

Proposal

To make this easier to use, I suggest we add a "expand all" and "collapse all" buttons to every settings page that has collapsible panels.

@JobV, on the other hand, suggested that we could add a search bar for settings.

We still have to update all the docs, but at least we would make things easier to find.

cc/ @mydigitalself