📚 CI: add what and why

Author: mbercx

docs/ci.md

@@ -1,5 +1,54 @@
-# Continuous integration
+# Continuous Integration
 
-!!!warning "Under Construction"
+## What
 
-    The workshop organiser is still working hard on this page! 🤓
+Continuous Integration (CI) is the practice of automatically building, testing, and validating your code every time you make changes.
+Instead of manually running tests or checks before merging code, CI systems do this automatically.
+
+## Why
+
+In a sense, a good software developer is _lazy_.
+They dislike manual work, and want to automate as much as possible.
+CI allows you to stop doing all the checks/steps for every code change manually.
+The added benefit is that this is _much_ less error prone: e.g. your documentation will never be out of date because you forgot to deploy it.
+
+!!!example "Real-world scenario"
+
+    You're maintaining a Python package, and someone wants to make a change to your code.
+    Without CI, you'll have to test their changes locally: did they install/run the `pre-commit`?
+    Are the tests passing for the Python versions you support?
+    Ain't nobody got time for that!
+
+## How
+
+GitHub Actions is a CI/CD (Continuous Integration/Continuous Deployment) platform built into GitHub.
+It lets you define workflows that run automatically in response to events like pushing code, opening pull requests, or adding a release tag.
+
+A GitHub actions workflow is defined in a YAML file, stored in the `.github/workflows` directory.
+Let's have a look at `deploy-mkdocs.yaml`:
+
+```yaml {.no-copy}
+# deploy-mkdocs.yaml
+name: Deploy MkDocs site to Github pages
+on:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: write
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: 3.x
+
+      - run: pip install mkdocs mkdocs-material
+      - run: mkdocs gh-deploy --force
+
+```