manual: make the sidebar slightly wider.

From Furo's `_layout.scss`:

    // Because CSS variables can't be used in media queries. The fact that this
    // makes the layout non-user-configurable is a good thing.

This is of course bullshit. All it means is that whenever anyone
upgrades Furo they have to manually build it with the following patch:

    $sidebar-width: 16em;

and then manually put `furo.css` into `_static/styles/`. I don't like
being told what (not) to do.

docs/manual/pyproject.toml

@@ -6,6 +6,12 @@ dependencies = [
     "sphinx-autobuild",
     "sphinx-inline-tabs",
     "sphinx-copybutton",
+    # If you change the version of Furo, check out the repository, apply the following patch
+    # to `src/furo/assets/styles/variables/_layout.scss`:
+    #   -$sidebar-width: 15em;
+    #   +$sidebar-width: 16em;
+    # then rebuild it and place the build product `src/furo/theme/furo/static/styles/furo.css`
+    # into `docs/manual/src/_static/styles/furo.css`.
     "furo",
 ]