For local development you need to change the antora configuration in antora-playbook.yml, and we don't have that documented anywhere.

Steps to reproduce:

1. Create a change in any file
2. Run the npm run build command
3. See that you don't see the change in the build site

As a workaround, you can make this change in your local antora-playbook.yml (without committing the changes):

diff --git a/antora-playbook.yml b/antora-playbook.yml
index 41fcc02..3210fd7 100644
--- a/antora-playbook.yml
+++ b/antora-playbook.yml
@@ -4,12 +4,8 @@ site:
   start_page: v0.27@en:ROOT:index.adoc
 content:
   sources:
-    - url: https://github.com/decidim/documentation
+    - url: .
       start_path: en
-      branches:
-        - release/0.27-stable
-        - release/0.26-stable
-        - develop
       edit_url: "https://github.com/decidim/documentation/edit/{refname}/{path}"
     - url: https://github.com/decidim/decidim
       start_path: docs

The solution would involve having two playbook configurations, improving the documentation and explaining a bit how antora works, and having two commands: npm run build and npm run prod-build or something like that.