feat: add X link and first blog post

[corylanou]

Summary

• Add @hype_markdown X/Twitter link to header and footer nav
• Add first blog post: "Single-Source Documentation" — a tutorial on using Hype to maintain one Markdown source for README and website docs

Test plan

• Verify X link appears in nav and footer, opens correct profile
• Verify blog post renders at /single-source-docs/
• Verify post appears in article list on homepage
• hype blog build succeeds with no warnings (13 articles)

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Use literal code fences in embedded hype.md example

The hype.md example escapes backticks as ` inside a fenced markdown block, so readers will see/copy the entity text instead of real triple backticks when following the tutorial. In CommonMark-style renderers, entities are not decoded inside code blocks, so this produces an invalid nested fence in the copied file and the install snippet won’t render as intended in the generated README.

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