Add a `docs/docker-compose.yml` file to run the public `shadcn` site

[pirate]

• https://github.com/Monadical-SAS/selfhostyourtech/blob/main/docs/docker-compose.yml
• https://github.com/Monadical-SAS/selfhostyourtech/blob/main/.github/workflows/static.yml

Subtasks

• build the shadcn public site to static html under docs/out and commit the output
• Update the CI build to rebuild the docs/out static html when the source changes
• Update the CI build to deploy the docs/out folder to selfhostyour.tech Github Pages
• add a simple nginx or python3 -m http.server -d . -b 0.0.0.0 80 service to docs/docker-compose.yml so that the demo server/user installs also serve the static docs site on port 80 by default