header choice should be constrained

[Erwan-le-Gall]

label: accessibility

When I insert a new header

Then I should be constrained to:

• immediately preceding higher level
• same level
• immediately following loxer level

because headers hierarchy should not have "holes" in it (it must be continuous).

This is a requirement for accessibility compliance.

[Erwan-le-Gall]

ping @cyberbaloo