Document every DNS + codegen plugin on the website

Add a "Plugins" section to the docs with two subsections — "DNS Providers" and "Code Generators" — and one page per plugin. Each DNS page covers: install command, options-schema fields, provider-specific account setup (IAM policy for route53, token scopes for cloudflare, per-domain API toggle for porkbun, tier requirement for godaddy, IP whitelist + replace-everything caveat for namecheap, service-account roles + ADC fallback for google), interactive + non-interactive login invocations, and documented quirks. Each codegen page covers: usage, the full type-mapping table, field-syntax / optional-vs-required handling, doc-comment preservation, and known gaps. Template changes: page.html and section.html both render a two-level nested sidebar (docs section → subsection → subsubsection), with clickable titles at each level. Previously only one level of nesting was supported. A small SCSS block adds nav-subsection styles that match the existing nav-section look at a deeper indent. 37 total pages in the docs tree, up from 26.