Move installation.md and rename administration.md to deploy.md under
docs/administration/, delete docs/guides/, and add a scope-only
README.md per the docs-README rules. Update README.md, SUPPORT.md,
and scripts/tests/environment/README.md cross-links.
Refresh roles/web-app-decidim/README.md to match the current OIDC
patch set (decidim-core omniauth.rb + omniauth_helper.rb only, no
secrets.yml on Rails 7.2, ENV-prefixed credential names).
