feat: Custom GPTs page, gitignore configs, SEO README, cross-references

- Gitignore individual GPT config files (custom-gpt/*-gpt.md) — instructions
  should not be publicly exposed
- Rewrite custom-gpt/README.md with SEO-optimized descriptions and live
  ChatGPT links for all 6 GPTs
- New docs page: docs/custom-gpts.md with grid cards, comparison table,
  and stats
- Add Custom GPTs to mkdocs.yml navigation
- Cross-reference Custom GPTs from:
  - Homepage (new grid card)
  - Getting Started (FAQ entry)
  - Personas page (try in ChatGPT callout)
  - Plugins page (FAQ entry)

Live GPT links:
- Solo Founder: chatgpt.com/g/g-69b315...
- SEO Audit Expert: chatgpt.com/g/g-69b3b0...
- Content Strategist: chatgpt.com/g/g-69b3af...
- Product Manager Toolkit: chatgpt.com/g/g-69b32c...
- Conversion Copywriter: chatgpt.com/g/g-69b327...
- CTO Advisor: chatgpt.com/g/g-69b326...

docs/plugins/index.md

@@ -464,3 +464,6 @@ domain-name/
 
 ??? question "What is ClawHub?"
     [ClawHub](https://clawhub.com) is the public registry for Claude Code plugins. Think of it like npm for AI agent skills. The `cs-` prefix is used only when a plugin slug conflicts with another publisher.
+
+??? question "Can I use skills without installing anything?"
+    Yes. We have [6 Custom GPTs for ChatGPT](../custom-gpts.md) that package Agent Skills into a conversational interface — no installation, no API keys. Just click and chat.