Add README and index.yaml for DCMS folders (#649) (#1647)

Documents the DCMS parameters and variables folders to match the
convention used by other gov/* departments (e.g. DfT, DfE, DHSC):
- expand the parameters/gov/dcms README from the "DCMS" acronym to
  the full "Department for Culture, Media and Sport"
- add parameters/gov/dcms/index.yaml so the department shows up with
  a label/description in the parameter tree metadata
- add a matching variables/gov/dcms README

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vahid-ahmadi

changelog.d/649.md

@@ -0,0 +1 @@
+- Add README and `index.yaml` metadata for the DCMS parameters and variables folders.

policyengine_uk/parameters/gov/dcms/README.md

@@ -1 +1 @@
-# DCMS
+# Department for Culture, Media and Sport

policyengine_uk/parameters/gov/dcms/index.yaml

@@ -0,0 +1,3 @@
+metadata:
+  label: DCMS
+  description: Department for Culture, Media and Sport

policyengine_uk/variables/gov/dcms/README.md

@@ -0,0 +1 @@
+# Department for Culture, Media and Sport