kyaml: refactor command documentation into .md files from go files

No new documentation added.

cmd/config/docs/commands/grep.md

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+## grep
+
+Search for matching Resources in a directory or from stdin
+
+### Synopsis
+
+  Search for matching Resources in a directory or from stdin.
+
+  QUERY:
+    Query to match expressed as 'path.to.field=value'.
+    Maps and fields are matched as '.field-name' or '.map-key'
+    List elements are matched as '[list-elem-field=field-value]'
+    The value to match is expressed as '=value'
+    '.' as part of a key or value can be escaped as '\.'
+
+  DIR:
+    Path to local directory.
+
+### Examples
+
+    # find Deployment Resources
+    kyaml grep "kind=Deployment" my-dir/
+    
+    # find Resources named nginx
+    kyaml grep "metadata.name=nginx" my-dir/
+    
+    # use tree to display matching Resources
+    kyaml grep "metadata.name=nginx" my-dir/ | kyaml tree
+    
+    # look for Resources matching a specific container image
+    kyaml grep "spec.template.spec.containers[name=nginx].image=nginx:1\.7\.9" my-dir/ | kyaml tree