Add SaaS and local government examples to domains.md

[southpawriter02]

Summary

The domains page currently has one example (restaurants). This PR adds two
more — a SaaS product (fictional "Beacon Analytics") and a local government
site (fictional "City of Maplewood") — to show how llms.txt can work
outside of developer tooling and open source documentation.

Each example includes:

• A complete llms.txt following the spec
• Brief guidance on domain-specific considerations
• A short note after the example highlighting what makes it different
  from the others

The SaaS example also includes a supplementary API reference page
(paralleling how the restaurant example includes a Franklin's BBQ menu)
to show what a linked .html.md resource might look like for a product
with a developer-facing API.

Why these two domains

The spec already mentions both as use cases — "development environments,
where LLMs need quick access to programming documentation and APIs" and
"breaking down complex legislation for stakeholders" — but the domains
page didn't have examples for either. SaaS is where most of the real-world
adoption is happening, and local government is an interesting test of the
format because it serves a non-technical audience navigating unfamiliar
bureaucratic structure.

A note on prose density

The restaurant section is pure show-don't-tell — one sentence of intro,
then the example. These two sections have a bit more prose around them
(a guidance paragraph before each example and a short note after), because
SaaS and government sites involve more structural decisions than a menu.

That said, I'd be happy to trim the commentary if a leaner approach
would better match the direction of the page. The examples themselves
are the real contribution — the prose is there to help them land, not
to editorialize.

@jph00