Added documentation for LDAP (#658)

* Added documentation for LDAP

Related to #133
Inspired by https://github.com/jenkinsci/kubernetes-operator/issues/133#issuecomment-555976832

website/content/en/docs/Getting Started/latest/ldap.md

@@ -0,0 +1,128 @@
+---
+title: "LDAP"
+linkTitle: "LDAP"
+weight: 9
+date: 2021-09-30
+description: >
+    Additional configuration for LDAP
+---
+
+Configuring LDAP is not supported out of the box, but can be achieved through
+plugins and some well tuned configurations.
+
+The plugin we will use is: <https://plugins.jenkins.io/ldap/>
+
+> Note: This is an example of how LDAP authentication can be achieved. The LDAP
+> plugin is from a third-party, and there may be other alternatives that suits
+> your use case better. Use this guide with a grain of salt.
+
+## Requirements
+
+- LDAP server accessible from the Kubernetes cluster where your Jenkins
+  instance will live.
+
+- Credentials to a manager account in your AD. Jenkins Operator will use
+  this account to authenticate with Jenkins for health checks, seed jobs, etc.
+
+## Steps
+
+In your Jenkins configuration, add the following plugin:
+
+```yaml
+plugins:
+    # Check https://plugins.jenkins.io/ldap/ to find the latest version.
+  - name: ldap
+    version: "2.7"
+```
+
+Easiest step is to then start up Jenkins then navigate to your instance's
+"Configure Global Security" page and configure it accordingly.
+
+`http://jenkins.example.com/configureSecurity/`
+
+Once it's set up and tested, you can navigate to your JCasC page and export
+the LDAP settings.
+
+`https://jenkins.example.com/configuration-as-code/`
+
+Feed the relevant new settings into your Kubernetes ConfigMap for your JCasC
+settings.
+
+Here's a snippet of the LDAP-related configurations:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: jenkins-casc
+data:
+  ldap.yaml: |
+    jenkins:
+      securityRealm:
+        ldap:
+          configurations:
+            - displayNameAttributeName: "name"
+              groupSearchBase: "OU=Groups,OU=MyCompany"
+              groupSearchFilter: "(& (cn={0}) (objectclass=group) )"
+              inhibitInferRootDN: false
+              managerDN: "CN=Jenkins Admin,OU=UsersSystem,OU=UsersOther,OU=MyCompany,DC=mycompany,DC=local"
+              managerPasswordSecret: "${LDAP_MANAGER_PASSWORD}"
+              rootDN: "DC=mycompany,DC=local"
+              server: "MyCompany.local"
+              userSearch: "SamAccountName={0}"
+              userSearchBase: "OU=MyCompany"
+          disableMailAddressResolver: false
+          disableRolePrefixing: true
+          groupIdStrategy: "caseInsensitive"
+          userIdStrategy: "caseInsensitive"
+```
+
+> Note the use of `${LDAP_MANAGER_PASSWORD}` above. You can reference
+> Kubernetes secrets in your JCasC ConfigMaps by adding the following to your
+> Jenkins object:
+>
+> ```yaml
+> kind: Jenkins
+> spec:
+>   configurationAsCode:
+>     configurations:
+>       - name: jenkins-casc
+>     secret:
+>       # This here
+>       name: jenkins-casc-secrets
+> ```
+>
+> ```yaml
+> apiVersion: v1
+> kind: Secret
+> metadata:
+>   name: jenkins-cred-conf-secrets
+> stringData:
+>   LDAP_MANAGER_PASSWORD: <password-for-manager-created-in-ldap>
+> ```
+>
+> Schema reference: [v1alpha2.ConfigurationAsCode](./schema/#github.com/jenkinsci/kubernetes-operator/pkg/apis/jenkins/v1alpha2.ConfigurationAsCode)
+
+Finally you must configure the Jenkins operator to use the manager's
+credentials from the AD.
+
+This is because this procedure will disable Jenkins' own user database, and the
+Jenkins operator still needs to be able to talk to Jenkins in an authorized
+manner.
+
+Create the following Kubernetes secret:
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: jenkins-operator-credentials-<jenkins-cr-name>
+  namespace: <jenkins-cr-namespace>
+stringData:
+  user: <username-for-manager-created-in-ldap>
+  password: <password-for-manager-created-in-ldap>
+```
+
+> Note: Values in stringData do not need to be base64 encoded. They are
+> encoded by Kubernetes when the manifest is applied.
+