Handbook Plugin table of contents/heading relationship

[joedolson]

The Handbooks TOC generates a link pointing to the heading ID. Because the ID is attached to the heading, which is not focusable, this moves the visual focus, but not keyboard focus.

Solutions are either to move the ID from the heading to the associated anchor, or to add tabindex=-1 to the heading.

[samuelsidler]

I think I'd prefer to add tabindex=-1 rather than moving the anchor around.

[coffee2code]

In 949:

Handbook plugin: add tabindex=-1 to table of content headings to prevent disjoined visual/keyboard focus

fixes #687.
props joedolson.