Define shared content schema for writings, portfolio, bookshelf, blogs

Adds content/README.md documenting the TOML frontmatter contract each
content type must follow, plus one seed entry per type to lock the
schema in a runnable form. Bookshelf entries use a kind field ("book"
or "paper") so academic papers (DOI/arXiv id) are first-class
alongside ISBN-driven books, not bolted on later.

content/writings/example-first-post/index.md

@@ -0,0 +1,15 @@
++++
+title = "Why This Site Exists"
+description = "A short note on what this new site is, and why it's more than a tech blog."
+author = "andwati"
+date = 2026-08-30
+draft = true
+[taxonomies]
+tags = ["meta"]
+[extra]
+llms_description = "An introductory post explaining the purpose of this personal digital-legacy site."
++++
+
+This is a placeholder post establishing the `writings` content schema. It will
+be replaced once the design system and migration from the old Zola posts are
+ready.