feat: add explicit buttons to heading titles

[sorawee]

This commit adds two buttons to each heading title. These buttons are by default hidden, but they will appear on hovering on the heading titles.

The first button is a "link here" button, which adjusts the URL of the current page to include an anchor to link to that heading title, similar to how GitHub makes each heading title in README documents a link to add an anchor.

The second button is to view Scribble source and internal link. Previously, clicking heading titles serves this functionality, but without an indicator that heading titles can be clicked, the feature is not discoverable. So this commit makes a transition toward an explicit button.

Example screenshots:

Page title

Section title

[sorawee]

Still not perfect, but this should be a bit better.

(It can be made better, but that will require a percentage value that is "ugly" instead. This is my best attempt without resorting to using the ugly numbers.)

[mflatt]

I'll second @jryans's suggestion and rationale to show the buttons only when mousing over the title.

[sorawee]

I made the update as requested.

Since we now show the buttons only on hovering over the heading titles, I removed the adjustments to make the buttons smaller*, to help with discoverability.

[*] well, there's still a place that needs this adjustment, since it would be too large with the full size.

[mflatt]

Thanks for the update! This looks good to me.