add documentation for backup filters (#9967)

* add documentation for backup filters

Add user guide for fine grained backup filters with examples
from easy to advanced.

Signed-off-by: Adam Zhang <adam.zhang@broadcom.com>

* address review comments

- enhanced example 3, explain how each item got excluded
- enhanced example 8, explain the exact match rule, and how the
  ordering affecting namespace that has  multiple match patterns
- cross link to restore side design
- fix the error msg to be consistent with implemenation

Signed-off-by: Adam Zhang <adam.zhang@broadcom.com>

---------

Signed-off-by: Adam Zhang <adam.zhang@broadcom.com>

site/data/docs/main-toc.yml

@@ -33,6 +33,8 @@ toc:
         url: /enable-api-group-versions-feature
       - page: Resource filtering
         url: /resource-filtering
+      - page: Fine-Grained Backup Filters
+        url: /fine-grained-backup-filters
       - page: Namespace glob patterns
         url: /namespace-glob-patterns
       - page: Backup reference