Add three free church themes for Eleventy, Jekyll, Astro and Hugo

[puikinsh]

Adds three free church website themes, each available for Eleventy, Jekyll,
Astro and Hugo — twelve entries.

Theme	For
Bethel	evangelical and Baptist congregations
Canterbury	Anglican, Episcopal and Catholic parishes
Threshold	church plants and multisite churches

They are twelve separate repositories rather than one, because each is a real
project in its own framework: an Eleventy user gets .eleventy.js, _data and
.njk layouts, a Hugo user gets a theme with theme.toml and an exampleSite,
and so on. Listing them per-SSG follows what the directory already does for
other multi-framework sets such as the Zerostatic Serif and Atlantic themes.

Every entry has been built from a clean clone and its output checked against the
original design, so the demos and the repositories match.

Two notes on the metadata:

• archetype: Business. There is no church or nonprofit archetype in the
  existing vocabulary, and Business is the closest to "an organisation's site"
  as distinct from a blog, portfolio or personal site. Happy to change it if you
  would rather add a new archetype or use Other.
• css: CSS. No framework — the styles are hand-written, and the typefaces
  are self-hosted and subset, so the themes make no external requests at all.

The demo links go to the theme demos, not to a business site.

[puikinsh]

Added a short body to each of the twelve entries — pages, how to start it, and the handful of things worth knowing — so the listing pages have something on them rather than front matter alone.

No images included: I can see scripts/build.js captures those from the demo URL itself, so they are yours to generate rather than mine to add. All twelve demos are live and respond to a headless browser, so testDemoUrls should not disable any of them.