Review and improve page template guidance

[EoinShaughnessy]

What

Review and improve page template guidance.

Why

While updating the guidance recently, we saw some things that could be improved.

For example, there's a heading that just reads 'Default', which might not tell the user much.

Also, the info under 'Changing template content' isn't organised very clearly. Maybe we could improve the nav by adding subheadings for setting variable and block options.

A full review will probably reveal other issues.

Who needs to work on this

Content designer or technical writer

Who needs to review this

Anyone in the team

Done when

• content designer or technical writer reviews guidance
• content designer or technical writer drafts update
• update passes review and is published