Preprocessor docs don't reuse frame

[Flamefire]

When opening https://www.boost.io/doc/libs/latest/libs/preprocessor/doc/ and clicking on the navigation on the left, the sub menu opens as expected.

Clicking the sub-item then opens a new window instead of changing the frame on the right.

Also the "back" link after opening a submenu also opens a new page with only the navigation

Might be related to #621

[rbbeeston]

Thanks @Flamefire , I'm reopening #621 as the issue affects more than just Preprocessor.