Reorganize docs into 4 group subdirectories with clean file names

- Group files into installation/, configuration/, features/, faq/
- Rename files without group prefix (e.g. features/timer.mdx, configuration/ui.mdx)
- Fix heading levels: title as H1, content starts at H2
- Normalize sub-field headings in configuration/file.mdx
- Add style.css for font size adjustment

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

style.css

@@ -0,0 +1,3 @@
+html {
+  font-size: 15px;
+}