== Background
Follow up work to T359001
== User story
As a reader on office wikimedia and other projects using Parsoid I want to be able to collapse sections.
== Requirements
[] JavaScript/CSS has been added that allows sections to be collapsed or expanded.
[] The code is guarded so it only applies to Parsoid views.
[] The sections are expanded by default (pending no decision on T363933 we will do the simplest possible thing here in the mean time)
[] Section collapsing has appropriate semantics indicating the expanded/collapsed status of a section, and that the header is a button. Reference the original accessibility implementation of section collapsing to confirm
Note: Remembering collapsed/expanded sections on back button is not in scope for this ticket.
== QA steps
[] If I view a page with sections, I can expand and collapse sections by clicking on the heading or the up/down arrow.
[] I can use "Find in page" on text in collapsed sections.
== Rollback plan
None necessary due to feature flag.
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