docs: added deep links doc

docs/user-guide/deep_links.md

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+# Deep Links
+
+Deep links allow users to quickly redirect to third-party systems such as Splunk, DataDog etc. from the Argo CD
+user interface.
+
+Argo CD administrator will be able to configure links that redirect users to third-party systems by providing 
+deep link templates configured in the `argocd-cm`. The templates can be conditionally rendered and are be able 
+to reference different types of resources relating to where the links show up, this includes projects, applications,
+or individual resources(pods, services etc.).
+
+## Configuring Deep Links
+
+The configuration for Deep Links is present in `argocd-cm` as `<location>.links` fields where 
+`<location>` determines where it will be displayed. The possible values for `<location>` currently are :
+- `project` : all links under this field will show up in the project tab in the Argo CD UI.
+- `application` : all links under this field will show up in the application summary tab.
+- `resource` : all links under this field will show up in the resource(deployments, pods, services etc.)
+  summary tab.
+
+Each link in the list has five subfields :
+1. `title` : title/tag that will be displayed in the UI corresponding to that link
+2. `url` : the actual URL where the deep link will redirect to, this field can be templated to use data from the
+   corresponing application, project or resource objects (depending on where it is located). This uses [text/template](pkg.go.dev/text/template) pkg for templating.
+3. `description` (optional) : a description for what the deep link is about
+4. `icon.class` (optional) : a font-awesome icon class to be used when displaying the links in dropdown menus.
+5. `if` (optional) : a conditional statement that results in either `true` or `false`, it also has access to the same
+   data as the `url` field. If the condition resolves to `true` the deep link will be displayed else it will be hidden. If
+   the field is omitted, by default the deep links will be displayed. This uses [antonmedv/expr](https://github.com/antonmedv/expr/tree/master/docs) for evaluating conditions.
+
+
+An example `argocd-cm.yaml` file with deep links and its variations :
+
+```yaml
+  # sample project level links
+  project.links: |
+    - url: https://myaudit-system.com?project={{.metadata.name}}
+      title: Audit
+      description: system audit logs
+      icon.class: "fa-book"
+  # sample application level links
+  application.links: |
+    # pkg.go.dev/text/template is used for evaluating url templates
+    - url: https://mycompany.splunk.com?search={{.spec.destination.namespace}}
+      title: Splunk
+    # conditionally show link e.g. for specific project
+    # github.com/antonmedv/expr is used for evaluation of conditions
+    - url: https://mycompany.splunk.com?search={{.spec.destination.namespace}}
+      title: Splunk
+      if: spec.project == "default"
+    - url: https://{{.metadata.annotations.splunkhost}}?search={{.spec.destination.namespace}}
+      title: Splunk
+      if: metadata.annotations.splunkhost
+  # sample resource level links
+  resource.links: |
+    - url: https://mycompany.splunk.com?search={{.metadata.namespace}}
+      title: Splunk
+      if: kind == "Pod" || kind == "Deployment"
+```