refactor: split the repo into self-contained validator, directory, quickstart, spec and site folders

validator/ holds mewslint (lint, css, fetch, DTD/CSS data); directory/ holds
the mews.page service (app, db, check, pages, cli) and depends on validator/;
quickstart/ holds mews-quickstart and md2mews.py for third parties who want
to create Mews content without cloning the repo; spec/ consolidates doc/ and
dtd/ with the canonical mews-0.1.css; site/ becomes a Pelican project with
the forward-facing pages as Markdown, the check form kept as a raw HTML
snippet. Root pyproject.toml is now a virtual uv workspace over validator,
directory and quickstart, with cross-cutting tests that span more than one
of them staying at the repo root. Makefile and flake.nix updated to match.

site/pyproject.toml

@@ -0,0 +1,13 @@
+[project]
+name = "mews-page"
+version = "0.1.0"
+description = "mews.page's own pages"
+requires-python = ">=3.10"
+dependencies = [
+    "pelican>=4.9",
+    "markdown>=3.4",
+    "pelican-mews",
+]
+
+[tool.uv.sources]
+pelican-mews = { path = "../pelican-mews" }