Add the dagger.#Plan core concept to Europa Docs

This is the entry point for all other core concepts.

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

docs/core-concepts/1202-plan.md

@@ -4,3 +4,76 @@ displayed_sidebar: europa
 ---
 
 # It all starts with a plan
+
+A CI/CD pipeline declared in Dagger starts with a plan, specifically `dagger.#Plan`
+
+This plan is the entrypoint for everything that runs within a pipeline.
+
+The simplest plan will have at least one input - the source code - and a few actions, usually build, test & deploy.
+This is our **Getting Started** example app plan structure:
+
+```cue
+dagger.#Plan & {
+  inputs: {
+    directories: app: path: "./"
+    // ...
+  }
+  actions: {
+    build: yarn.#Build & {
+      // ...
+    }
+    test: yarn.#Run & {
+      // ...
+    }
+    // ...
+  }
+}
+```
+
+When the above plan gets executed via `dagger up`, it produces the following output:
+
+```shell
+dagger up dev.cue
+[✔] inputs.directories.app                                        0.1s
+[✔] actions.build                                                 0.6s
+[✔] actions.test                                                  0.6s
+```
+
+Since these actions have run before, they are cached and take less than 1 second to complete.
+
+While the names used for the actions above - `build`, `test` - are short & descriptive,
+any other names would have worked. Put differently, action naming does not affect plan execution.
+
+In the example above, the `build` action is an instance of the yarn package build definition.
+This is written as `build: yarn.#Build`
+
+Default definition configuration can be modified via curly brackets, e.g.
+
+```cue
+  actions: {
+    build: yarn.#Build & {
+      // ...
+    }
+```
+
+We can build complex pipelines efficiently by referencing any definition, from any package in our actions.
+This is one of the fundamental concepts that makes Dagger a powerful devkit for CI/CD.
+Before we can use a package in a plan, we need to declare it at the top of the pipeline configuration, like this:
+
+```cue
+import (
+  "universe.dagger.io/yarn"
+)
+```
+
+Since we are using the plan definition from the dagger package - `dagger.#Plan` - we also need to declare it at the top of the pipeline configuration:
+
+```cue
+import (
+  "dagger.io/dagger"
+  "universe.dagger.io/yarn"
+)
+```
+
+Now that we understand the basics of a Dagger plan, we are ready to learn more about inputs and how to configure them.
+This will enable us to configure plans just-in-time, which is something that typically happanes on every CI run.