doc: add "Developer documentation" page

Add a manual for enabling new kernel tree in
the newly added page for Developer documentation.

Signed-off-by: Jeny Sadadia <[email protected]>

doc/developer-documentation.md

@@ -0,0 +1,52 @@
+---
+title: "Developer Documentation"
+date: 2024-05-22
+description: "KernelCI API/Pipeline developer manual"
+weight: 6
+---
+
+## Enabling a new Kernel tree
+
+We can monitor different kernel trees in KernelCI.
+This manual describes how to enable them in [`kernelci-pipeline`](https://github.com/kernelci/kernelci-pipeline.git).
+
+
+### Pipeline configuration
+The pipeline [configuration](https://github.com/kernelci/kernelci-pipeline/blob/main/config/pipeline.yaml) file has `trees` section.
+In order to enable a new tree, we need to add an entry there.
+
+```yaml
+trees:
+    <tree-name>:
+        url: "<tree-url>"
+```
+For example,
+```yaml
+trees:
+    kernelci:
+        url: "https://github.com/kernelci/linux.git"
+```
+
+After adding a `trees` entry, we need to define build configurations for it.
+In the same [configuration](https://github.com/kernelci/kernelci-pipeline/blob/main/config/pipeline.yaml) file, `build_configs` section is there to specify them.
+For example, we need to specify which branch to monitor of a particular tree and other build variants as well.
+
+For instance,
+```yaml
+build_configs:
+
+    kernelci_staging-mainline:
+        tree: kernelci
+        branch: 'staging-mainline'
+        variants:
+            gcc-10:
+                build_environment: gcc-10
+                architectures:
+                    x86_64:
+                        base_defconfig: 'x86_64_defconfig'
+                        filters:
+                            - regex: { defconfig: 'x86_64_defconfig' }
+```
+
+That's it! The tree is enabled now.
+All the jobs defined under `jobs` section of [config file](https://github.com/kernelci/kernelci-pipeline/blob/main/config/pipeline.yaml) would run on the newly added tree until specified otherwise.