Disallow edits on System.Text.RegularExpressions docs (#12529)

The source of truth for System.Text.RegularExpressions XML docs is
being moved to dotnet/runtime (see dotnet/runtime#127038).

- Add System.Text.RegularExpressions to disallow-edits.yml GitOps policy
- Set open_to_public_contributors to false in docfx.json

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: danmoseley

.github/policies/disallow-edits.yml

@@ -67,6 +67,9 @@ configuration:
               - filesMatchPattern:
                   matchAny: true
                   pattern: xml/System.Runtime.Serialization/XsdDataContractExporter.xml
+              - filesMatchPattern:
+                  matchAny: true
+                  pattern: xml/System.Text.RegularExpressions/*
         then:
             - addReply:
                 reply: >-

docfx.json

@@ -174,6 +174,7 @@
         "api/System.Net.ServerSentEvents.**": false,
         "api/System.Numerics.Tensors.**": false,
         "api/System.Reflection.Context.**": false,
+        "api/System.Text.RegularExpressions.**": false,
 
         "api/System.Diagnostics.FileVersionInfo.yml": false,
         "api/System.Linq.AsyncEnumerable.yml": false,