docs: update policy types, add docs for expression policies

docs/policies/expression/index.md

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+# Expression Policy
+
+Expression Policies allows you to write custom Policy Logic using Jinja2 Templating language.
+
+For a language reference, see [here](https://jinja.palletsprojects.com/en/2.11.x/templates/).
+
+The following objects are passed into the variable:
+
+- `request`: A PolicyRequest object, which has the following properties:
+    - `request.user`: The current User, which the Policy is applied against. ([ref](../../property-mappings/reference/user-object.md))
+    - `request.http_request`: The Django HTTP Request, as documented [here](https://docs.djangoproject.com/en/3.0/ref/request-response/#httprequest-objects).
+    - `request.obj`: A Django Model instance. This is only set if the Policy is ran against an object.
+- `pb_is_sso_flow`: Boolean which is true if request was initiated by authenticating through an external Provider.
+- `pb_is_group_member(user, group_name)`: Function which checks if `user` is member of a Group with Name `gorup_name`.
+
+There are also the following custom filters available:
+
+- `regex_match(regex)`: Return True if value matches `regex`
+- `regex_replace(regex, repl)`: Replace string matched by `regex` with `repl`