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...

.gitignore

@@ -35,6 +35,9 @@ AUDIT_REPORT.md
 # Archive folder (historical/backup files)
 archive/
 
+# Custom GPT configurations (private — instructions not publicly exposed)
+custom-gpt/*-gpt.md
+
 # MkDocs build output
 site/
 .playwright-mcp/