Summary:
D14093 will add back middle-clicking to toggle between {nav Fit} (with
{key F} as its keyboard shortcut) and {nav 100%} zoom. In addition,
D14094 sets {key Shift F} as a shortcut for {nav Fill}.

Going along with this, here we add {key Shift}-middle-clicking for
{nav Fill}. The code is moved to `DocumentView` in order not to
duplicate the handling of the toggling behaviour.

We also adapt the docbook accordingly, as some users still seem to read
the manual.

CCBUG: 396360

Depends on D14093

Test Plan:
Middle-click multiple times on an image in {nav View} mode and observe
{nav Fit} is toggled. Repeat while holding down {key Shift} for
{nav Fill}.
Proof-read docbook ("View mode" and "Tips" sections).

Reviewers: #gwenview, muhlenpfordt

Reviewed By: #gwenview, muhlenpfordt

Subscribers: muhlenpfordt, kde-doc-english

Tags: #documentation

Differential Revision: https://phabricator.kde.org/D14103