Fix On This Page algorithm

[SIGSTACKFAULT]

scope-creep edition!

• sort by the actual Y position. and not the top border width.
• sort in the correct direction
• handle the case where there are no <h2>s (which is the case on migration guides)
• compare <a href> which need to be there anyway instead of <li data-fragment>

[TrialDragon]

LGTM. Tho why do we use <h3> instead of <h2> as the next step down from a <h1> in the migration guides I wonder. Semantically weird, and style-wise a <h2> can be styled to look correctly.

[IceSentry]

Tho why do we use <h3> instead of <h2>

The code from that is pretty old, so my guess is just that cart liked the look of a h3 better and nobody questioned it since, but yeah, we should just use h2 and update the styling

[SIGSTACKFAULT]

scope creep >.<

edit: re-requesting because, well, i added more stuff. updated description.

[SIGSTACKFAULT]

Will rebase when I get home tomorrow

[SIGSTACKFAULT]

Also request from @Jondolf because after rebasing i had to mess with your styles

[alice-i-cecile]

@SIGSTACKFAULT once merge conflicts are resolved I'll merge this in for you.

[SIGSTACKFAULT]

@alice-i-cecile resolved!