Missing docs

[jbmoelker]

• docs/architecture.md: add more detail, see comment (moved to separate issue #212)
• [x] docs/a11y.md: linting, skip links, required alt texts, semantics, native elements with built-in a11y, css helpers; (see #99)
• [x] docs/getting-started.md: use template, create Cloudflare and DatoCMS instance, configure; (see #90)
• docs/icons.md: directory, sprite, component (see project structure and icon component for now) (moved to separate issue #210)
• [x] docs/seo.md: DatoCMS SEO preferences, SEO fields, SEO head; (see #75)
• [x] docs/blocks-and-components.md (see #79)
• [x] docs/cms-content-modelling.md (see #88)
• [x] docs/cms-data-loading.md (see #78)
• [ ] docs/project-structure.md: update with path aliases, and conventions (.client.ts, .fragment|query.graphql, .partial.astro, /api/...), mention create:* scripts (see #82).
• [x] docs/routing.md: nested, api, 404 pages & middleware, i18n, redirects (#64)
• [x] docs/upgrade.md: (see #191), (linking migrations is moved to separate issue #211)

[jbmoelker]

A more realistic architecture diagram than the one in the main readme:

But Mermaid makes it look kinda messy. So need to figure something else out.