docs: the settings file has no reference page — `mesh:` and `airfoil:` are reachable only through docstrings

[1-Bort-1]

Found while checking that the docs describe what v4.4.0 ships (#276).

The API reference is complete: MeshSettings and AirfoilSettings are exported
and rendered on docs/src/types.md:31-32, and the helpers that turn a block into
arguments (airfoil_solver, alpha_range, delta_range, reynolds,
rotation_matrix, slice_args, preview_args) are on
docs/src/functions.md:119-129. What is missing is anywhere a reader can see the
file itself:

• No .md page carries a sample vsm_settings.yaml or a literal mesh: /
  airfoil: block, and docs/make.jl lists no settings page.
• solver: xfoil — choosing the backend for a whole dataset from the settings
  file — appears on no .md page. docs/src/airfoil_pipeline.md:56 documents
  only the keyword route, obj_to_yaml's aero_solver argument.
• README.md never mentions settings files. Its one pointer is README.md:98,
  "Wing geometry can also be loaded from YAML files or .obj files. See the
  examples for details."

This predates v4.4.0 rather than being introduced by it, so it did not hold the
release up.

What I would do: one docs/src/settings.md showing an annotated
vsm_settings.yaml with a wing's mesh: and airfoil: blocks, added to the
docs/make.jl page list, and one README line pointing at it.

[1-Bort-1]

Adding docs/src/settings.md: an annotated vsm_settings.yaml showing a wing's mesh: and airfoil: blocks, including solver: xfoil, listed in docs/make.jl and pointed at from README.md:98. Docs only -- no src/ change, no CHANGELOG entry (no behaviour change, and #302 holds the changelog). Not touching the existing API reference pages.