search docs: collapsible regex mode section

Created by: rvantonder

Stacked on https://github.com/sourcegraph/sourcegraph/pull/41289

This is a concept idea to make the doc explanation of regex mode quirks collapsible, please leave thoughts.

Reason: "Regular expression mode" is largely superseded by supporting /.../ syntax in standard mode, and the docs take up a lot of space to basically explain the very quirky behaviors associated with it. I think we can do with something to make it less prominent.

https://user-images.githubusercontent.com/888624/188237211-3ab68229-d65b-463b-b867-d04ca2bb9e00.mp4

Test plan

Docs