html renderer: correct the button group

[sorawee]

This PR fixes a couple of issues related to the button group.

• On #lang scribble/base, only show the "Link to here" button, and do so with correct styling.
• Make it possible for the button group to exceed the right margin to preserve the previous HTML layout.
• Place anchor at the beginning of the headings, not at the end, so that on navigation, we will see the full heading.

Thanks to @mflatt who reported some of the issues above and helped with testing.

Fixes https://github.com/racket/scribble/issues/381

[mflatt]

These changes look great to me. They make my class web page as a Scribble document render nicely again, and they fix issues in the documentation, like an extra blank(-seeming) line after the title of the Scribble documentation. Thanks!