Adjust API docs side bar dimensions

The dimensions of the side bar navigation added in JDK-8320458 were chosen to accommodate a wide range of content, ranging from very short to very long TOC entry names. This somehow led to the spacing of the side bar to reflect the "worst case" scenario where a class has very long member names.

However, based on informal feedback and our own judgement, the side bar often looks a bit oversized with average class content. On the other side, longer entries are not that problematic as they will either be wrapped or truncated.

The intention is therefore to slightly reduce the width of the TOC side bar, leaving more space for the main content of the page.