docs: say what solid-node is on the documentation homepage

The homepage opened straight into its toctrees, so a reader arriving from
a search result or a link had to pick a page and infer the project from
it. "Why Solid Node" carries the positioning argument, but it is one
entry down and answers a different question than "what is this".

The lead now states the framework in one paragraph: a Python node tree
over the existing CAD backends, incremental rebuilds so a growing project
keeps moving, a live browser viewer, mechanical assertions before
printing, and the Apache-2.0 terms. It restates what README.rst and the
package description already say rather than making a new claim.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: lfagundes

docs/index.rst

@@ -1,6 +1,18 @@
 Solid Node documentation
 ========================
 
+Solid Node is an Open Source Python framework for parametric, 3D-printable
+mechanical projects. You describe a machine as a tree of nodes in Python —
+leaf parts modelled with the CAD backend that suits them, whether OpenSCAD,
+SolidPython or CadQuery, assembled and animated through rotations and
+translations — and the framework builds only the pieces that changed, so a
+project keeps moving as it grows past the point where a monolithic render
+becomes too slow. A live viewer in the browser reflects each edit as you
+save it, and mechanical assertions let you test how components fit and move
+before committing anything to the printer. It is released under the Apache
+License 2.0: build what you like with it, and license your own designs
+however you choose.
+
 .. toctree::
    :maxdepth: 2
    :caption: Getting started