Crosslink to and from namespaces information

I had trouble finding the core information about namespaces when
working on another page, so this commit improves cross-linking to
and from this information.

doc/api/namespaces.md

@@ -6,7 +6,8 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # Namespaces API
 
-Usernames and group names fall under a special category called namespaces.
+Usernames and group names fall under a special category called
+[namespaces](../user/group/index.md#namespaces).
 
 For users and groups supported API calls see the [users](users.md) and
 [groups](groups.md) documentation respectively.

doc/development/namespaces_storage_statistics.md

@@ -14,7 +14,7 @@ storage consumed by a group, and allow easy management.
 
 ## Proposal
 
-1. Create a new ActiveRecord model to hold the namespaces' statistics in an aggregated form (only for root namespaces).
+1. Create a new ActiveRecord model to hold the namespaces' statistics in an aggregated form (only for root [namespaces](../user/group/index.md#namespaces)).
 1. Refresh the statistics in this model every time a project belonging to this namespace is changed.
 
 ## Problem

doc/user/group/index.md

@@ -704,6 +704,7 @@ The group's new subgroups have push rules set for them based on either:
 - [Lock the sharing with group feature](#prevent-a-project-from-being-shared-with-groups).
 - [Enforce two-factor authentication (2FA)](../../security/two_factor_authentication.md#enforcing-2fa-for-all-users-in-a-group): Enforce 2FA
   for all group members.
+- Namespaces [API](../../api/namespaces.md) and [Rake tasks](../../raketasks/features.md)..
 
 ## Troubleshooting