docs: add split and lower built-in examples (#8992)

Another small pass on #3786.

Adds playground examples for:

- `split` — pull a team segment out of a path
- `lower` — normalize mixed-case roles before an allowlist check

Same layout as the other string examples. Eval'd with opa 1.19 against
the sample input/data.

---------

Signed-off-by: Dean Chen <51218137+locker95@users.noreply.github.com>
Signed-off-by: Dean Chen <862469039@qq.com>

docs/docs/policy-reference/_examples/strings/lower/normalize-role/config.json

@@ -0,0 +1,6 @@
+{
+  "showInput": true,
+  "showData": true,
+  "showTitles": false,
+  "titleSize": 4
+}

docs/docs/policy-reference/_examples/strings/lower/normalize-role/data.json

@@ -0,0 +1,3 @@
+{
+  "admin_roles": ["admin", "owner"]
+}

docs/docs/policy-reference/_examples/strings/lower/normalize-role/input.json

@@ -0,0 +1,3 @@
+{
+  "role": "Admin"
+}

docs/docs/policy-reference/_examples/strings/lower/normalize-role/intro.md

@@ -0,0 +1,4 @@
+<!-- markdownlint-disable MD041 -->
+
+`lower` returns the lowercase form of a string. Use it when identity providers
+send mixed-case roles or emails and you want a stable comparison.

docs/docs/policy-reference/_examples/strings/lower/normalize-role/output.json

@@ -0,0 +1,4 @@
+{
+  "allow": true,
+  "role": "admin"
+}

docs/docs/policy-reference/_examples/strings/lower/normalize-role/policy.rego

@@ -0,0 +1,7 @@
+package play
+
+role := lower(input.role)
+
+default allow := false
+
+allow if role in data.admin_roles

docs/docs/policy-reference/_examples/strings/lower/normalize-role/title.txt

@@ -0,0 +1 @@
+Normalize roles with lower

docs/docs/policy-reference/_examples/strings/split/team-from-path/config.json

@@ -0,0 +1,6 @@
+{
+  "showInput": true,
+  "showData": false,
+  "showTitles": false,
+  "titleSize": 4
+}

docs/docs/policy-reference/_examples/strings/split/team-from-path/data.json

@@ -0,0 +1 @@
+{}

docs/docs/policy-reference/_examples/strings/split/team-from-path/input.json

@@ -0,0 +1,3 @@
+{
+  "path": "/teams/payments/deploy"
+}

docs/docs/policy-reference/_examples/strings/split/team-from-path/intro.md

@@ -0,0 +1,4 @@
+<!-- markdownlint-disable MD041 -->
+
+`split` breaks a string into a list of parts. Policies often use it to pull a
+segment out of a path or dotted name before comparing against a list of allowed values.

docs/docs/policy-reference/_examples/strings/split/team-from-path/output.json

@@ -0,0 +1,10 @@
+{
+  "allow": true,
+  "parts": [
+    "",
+    "teams",
+    "payments",
+    "deploy"
+  ],
+  "team": "payments"
+}

docs/docs/policy-reference/_examples/strings/split/team-from-path/policy.rego

@@ -0,0 +1,9 @@
+package play
+
+# "/teams/payments/deploy" -> ["", "teams", "payments", "deploy"]
+parts := split(input.path, "/")
+team := parts[2]
+
+default allow := false
+
+allow if team == "payments"

docs/docs/policy-reference/_examples/strings/split/team-from-path/title.txt

@@ -0,0 +1 @@
+Pull a path segment with split

docs/docs/policy-reference/builtins/strings.mdx

@@ -64,3 +64,17 @@ See also the note at the top of this page about how `sprintf` pre-processes
 values (for example with `%T`).
 
 <PlaygroundExample dir={require.context('../_examples/strings/sprintf/deny-message')} />
+
+### `split`
+
+`split` returns a list of substrings separated by a delimiter. Use it when a
+path, hostname, or other structured string needs to be broken into parts.
+
+<PlaygroundExample dir={require.context('../_examples/strings/split/team-from-path')} />
+
+### `lower`
+
+`lower` converts a string to lowercase. It is useful for normalisation and
+formatting when values may arrive in mixed case.
+
+<PlaygroundExample dir={require.context('../_examples/strings/lower/normalize-role')} />