Outline tree is quirky

[shakiba]

I noticed a couple of quirky behavior in outline:

• If the page should start with header level 1 or 2, otherwise outline will be empty
• If there are level 3 headers under level 1 (with no level 2), level 3 headers will not appear in outline
• If the page has two level 2 headers and many level 3 headers under first level 2, then the second level 2 will not appear (the workaround for this issue is to change level 2 to 1, and 3 to 2)