Possibility to add subtitles as a participant

Is your feature request related to a problem? Please describe.
For the participant entered records it has not been possible to add subtitles. This is for a good reason because of the weird heading orders we have in Decidim today.

Hopefully the redesign efforts would bring consistency to the heading orders, so I would suggest that after redesign is implemented, we would also allow adding subheadings for participants.

Describe the solution you'd like
I would like to have the possibility to add subtitles to my proposal body along with some other records.

An example would be this particular form I am filling right now, i.e. submitting a proposal to describe my feature idea. In this form we have several sections that would be better represented as subtitles.

The particular views at the participant side would be:

• Debates new/edit: debate description
• Initiatives new/edit: initiative description
• Meetings new/edit
• Meeting description
• Meeting registration terms
• Proposals new/complete/edit: proposal body

Describe alternatives you've considered
Customizing the editor for each instance but I would rather have this as a default feature if we can get over the headings order issue through redesign.

Additional context
This issue came up when reviewing PR 10673.

This feature should be only implemented for the redesigned layout along with the new TipTap editor.

Could this issue impact on users private data?
No.

Funded by
Can be further discussed. This shouldn't be too difficult to implement, so we can possibly fund this ourselves.