Documentation sidebar numbering skips 4: no section claims sort: 4

[philipcardiff]

Once #84 lands, the Documentation sidebar numbers its sections — and the
numbering visibly skips 4:

1. Overview
2. Solid models
3. Material models      <- arrives with #83
5. Fluid models
6. Fluid-solid coupling schemes: fluidSolidInterfaces
7. Function Objects
8. Applications
9. Under the hood: solids4FoamModels

No section claims sort: 4. It is a genuine gap, not a page that failed to
render.

Where the numbers come from

Every Documentation section index except overview.md is a symlink into the
imported/solids4foam submodule, so the sort: values are front matter in the
toolbox repository, not here:

Section	sort:	Source of the front matter
Overview	1	documentation/overview.md (this repo)
Solid models	2	src/solids4FoamModels/solidModels/README.md
Material models	3	src/solids4FoamModels/materialModels/README.md
—	4	unclaimed
Fluid models	5	src/solids4FoamModels/fluidModels/README.md
Fluid-solid interfaces	6	src/solids4FoamModels/fluidSolidInterfaces/README.md
Function objects	7	src/solids4FoamModels/functionObjects/README.md
Applications	8	applications/README.md
Under the hood	9	src/solids4FoamModels/README.md

The gap dates from solids4foam/solids4foam@fa8e7618, the commit that laid out
these values. Slot 3 was deliberately reserved for material-models/ and is
now filled by #83; slot 4 was never assigned to anything, and no README
anywhere in src/solids4FoamModels carries a section-level sort: 4.

Filing here because this is where the symptom is visible and where the sidebar
template lives, but note the fix is a cross-repository change: whichever option
is chosen below, the edits land in solids4foam/solids4foam and reach the site
through the usual submodule bump.

Options

1. Close the gap — renumber Fluid models 5→4, FSI 6→5, Function objects
   7→6, Applications 8→7, Under the hood 9→8. Contiguous, but leaves no room to
   insert a section later without renumbering again.
2. Claim slot 4 — if a section is planned to sit between material models
   and fluid models, give it sort: 4 when it is written. The toolbox has four
   top-level directories under src/solids4FoamModels with no README at all
   (dynamicFvMesh, higherOrderHelpers, numerics, physicsModel), so
   there are plausible candidates.
3. Leave it — accept a non-contiguous sidebar. Nothing is broken; only the
   presentation is odd.

Option 1 is the smaller change if no section is actually planned for slot 4.

Not urgent

Nothing is broken: ordering is correct and every page is reachable. This only
became visible because #84 started printing the numbers that ordering was
already derived from.

Related: #83, #84.

[philipcardiff]

The wider design question behind this — ordering metadata living in the toolbox READMEs as absolute numbers — is written up in #87. Whichever way #87 goes would make gaps like this one impossible by construction, so it may be worth resolving that first rather than renumbering by hand here.