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

No new documentation added.

cmd/config/docs/commands/merge.md

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+## merge
+
+Merge Resource configuration files
+
+### Synopsis
+
+Merge Resource configuration files
+
+Merge reads Kubernetes Resource yaml configuration files from stdin or sources packages and write
+the result to stdout or a destination package.
+
+Resources are merged using the Resource [apiVersion, kind, name, namespace] as the key.  If any of
+these are missing, merge will default the missing values to empty.
+
+Resources specified later are high-precedence (the source) and Resources specified
+earlier are lower-precedence (the destination).
+
+For information on merge rules, run:
+
+	kyaml docs merge
+
+### Examples
+
+    cat resources_and_patches.yaml | kyaml merge > merged_resources.yaml