Improve navbar on top page

[henrikt-ma]

This fixes two issues with the navbar (table of contents, shown as left sidebar) on the top page:

• The document title shouldn't be present in the navbar (only in page header, where it happens to be missing on the top page).
• Entire table of contents has excessive indentation under the document title item.

I suggest that this is also cherry-picked to maint/3.5, as it has an impact on the first impression of the document.

[HansOlsson]

Good!