Improve page structure

[StephDriver]

Aim

The have well structured pages

• H1 then H2, then H3 etc. consistent, sensible structure. Intuitive for the user to anticipate and navigate.
• landmarks, regions etc that make sense (no more 'favourite space images' on every page!)
• The headings themselves should be descriptive, and easy to distinguish.
• Tables, figures, exercises, (anything that could have a cross reference in the text) should be reflected in the heading structure.

Background

The structure of the pages are not consistent, most of this is about heading levels not being consistent, but regions and landmarks should also be reviewed.

Pages are formed from multiple templates being slotted together, so I propose the parent template pass the heading level to the child so that the heading structure will always be known at the parent level, rather than hard coded into any child templates.

This needs to be done across all themes.

Refer to #4194 for examples of page structure issues.

Consider:

WCAG 1.3.1 - Info and Relationships

Information, structure, and relationships conveyed through presentation can be programmatically determined or are available in text.

Techniques to meet 1.3.1

WCAG 2.4.6 - Headings and Labels

Headings and labels describe topic or purpose.

Technique G130 - Providing Descriptive Headings