Add front matter that informs Hugo that the Grafana content is versioned (#74259)

This is necessary to allow version inference in CI builds.
Shortcodes like `docs/reference` and `docs/shared` use version inference to avoid the writer having to hardcode specific versions or use relative paths within the documentation set.

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

.drone.yml

@@ -1061,6 +1061,8 @@ steps:
   name: lint-docs
 - commands:
   - mkdir -p /hugo/content/docs/grafana/latest
+  - 'echo -e ''---\nredirectURL: /docs/grafana/latest/\ntype: redirect\nversioned:
+    true\n---\n'' > /hugo/content/docs/grafana/_index.md'
   - cp -r docs/sources/* /hugo/content/docs/grafana/latest/
   - cd /hugo && make prod
   image: grafana/docs-base:dbd975af06
@@ -1347,6 +1349,8 @@ steps:
   name: lint-docs
 - commands:
   - mkdir -p /hugo/content/docs/grafana/latest
+  - 'echo -e ''---\nredirectURL: /docs/grafana/latest/\ntype: redirect\nversioned:
+    true\n---\n'' > /hugo/content/docs/grafana/_index.md'
   - cp -r docs/sources/* /hugo/content/docs/grafana/latest/
   - cd /hugo && make prod
   image: grafana/docs-base:dbd975af06
@@ -4212,6 +4216,6 @@ kind: secret
 name: gcr_credentials
 ---
 kind: signature
-hmac: c14ca5e053041018df6b66df642accd7e7eee7e2b43ba8c8f7dee6c446a41bb0
+hmac: 852af171d897f0a2cc0b03375fa8dfeacc65c2df7113c5efd0e21b03195dd7af
 
 ...

scripts/drone/steps/lib.star

@@ -788,6 +788,7 @@ def build_docs_website_step():
         "image": images["docs"],
         "commands": [
             "mkdir -p /hugo/content/docs/grafana/latest",
+            "echo -e '---\\nredirectURL: /docs/grafana/latest/\\ntype: redirect\\nversioned: true\\n---\\n' > /hugo/content/docs/grafana/_index.md",
             "cp -r docs/sources/* /hugo/content/docs/grafana/latest/",
             "cd /hugo && make prod",
         ],