feat: initialize marketing site from groombook/groombook docs/

Migrated marketing site content from groombook/groombook/docs/ to this
dedicated org-level GitHub Pages repo. Site will publish at groombook.github.io.

Files migrated:
- index.html (landing page)
- mobile-groomers.html
- blog/launch.html, blog/why-we-built.html
- getting-started/index.html
- _config.yml (updated with explicit url)

Refs: GRO-195

Co-Authored-By: Paperclip <noreply@paperclip.ing>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

_config.yml

@@ -0,0 +1,20 @@
+title: Groom Book
+description: Open source, self-hostable pet grooming business management and CRM.
+url: https://groombook.github.io
+theme: minima
+plugins:
+  - jekyll-seo-tag
+  - jekyll-feed
+defaults:
+  - scope:
+      path: ""
+    values:
+      layout: default
+footer:
+  links:
+    - label: GitHub
+      url: https://github.com/groombook/groombook
+    - label: Issues
+      url: https://github.com/groombook/groombook/issues
+    - label: Discord
+      url: https://discord.gg/groombook