Update the deployment instructions

Author: bbatsov

.gitignore

@@ -1,3 +1,5 @@
 # For vscode:
 .vscode
 node_modules
+# Build output
+build/

README.adoc

@@ -42,30 +42,18 @@ $ cd docs.rubocop.org
 $ make build
 ----
 
-TIP: You can preview your changes by opening `docs/index.html` in your favorite browser.
+TIP: You can preview your changes by opening `build/site/index.html` in your favorite browser.
 
-Afterwards you can publish your changes like this:
-
-[source]
-----
-$ git add docs
-$ git commit -m 'Update site docs'
-$ git push
-----
-
-You can also use `make` to publish changes in a single step:
-
-[source]
-----
-$ make deploy
-----
+If you want to make changes to the manual's page structure you'll have to edit
+link:https://github.com/rubocop/rubocop/blob/master/doc/modules/ROOT/nav.adoc[nav.adoc].
 
-It basically does `antora --fetch` and `git push`.
+== Deploying to GitHub Pages
 
 NOTE: You'll need commit access to the repository for this to work.
 
-If you want to make changes to the manual's page structure you'll have to edit
-link:https://github.com/rubocop/rubocop/blob/master/doc/modules/ROOT/nav.adoc[nav.adoc].
+The site is automatically deployed to GitHub pages using a GitHub Action.
+The action will be triggered by any push to the `master` branch.
+It can also be triggered manually if needed.
 
 == Using Docker
 
@@ -74,11 +62,9 @@ inside a Docker container like this:
 
 [source]
 ----
-$ docker run --rm -t -v $(pwd):/docs antora/antora:3.1.10 --fetch /docs/antora-playbook.yml
+$ docker run --rm -t -v $(pwd):/build/site antora/antora:3.1.10 --fetch /docs/antora-playbook.yml
 ----
 
-Then, proceed with `git add`, `git commit` and `git push` as mentioned above.
-
 == Updating the Playbook
 
 When cutting new releases you'll have to updated `antora-playbook.yml` to mention