Add comprehensive documentation for SnapForm features and workflows

- Introduced new guides for Depth Plane, Surface, and animation workflows.
- Added installation instructions and FAQs to assist users in setup and troubleshooting.
- Created a quick-start guide for first-time users to convert images to Depth Planes.
- Documented the process for removing backgrounds from images.
- Established a models and storage section to clarify data management.
- Included a stylesheets file for improved documentation styling.
- Updated mkdocs configuration for better navigation and theme settings.

docs/stylesheets/extra.css

@@ -0,0 +1,13 @@
+:root {
+  --md-text-font: "Open Sans", "Noto Sans SC", "Microsoft YaHei", sans-serif;
+  --md-code-font: "Cascadia Code", "SFMono-Regular", Consolas, monospace;
+}
+
+.md-typeset h1 {
+  font-weight: 700;
+  letter-spacing: -0.02em;
+}
+
+.md-typeset .grid.cards > ul > li {
+  border-radius: 0.5rem;
+}