Add docs for custom project templates

doc/user/admin_area/custom_project_templates.md

+# Custom instance-level project templates **[PREMIUM ONLY]**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/6860) in [GitLab Premium](https://about.gitlab.com/pricing) 11.2.
+
+When you create a new project, creating it based on custom project templates is
+a convenient option to bootstrap from an existing project boilerplate.
+The administration setting to configure a GitLab group that serves as template
+source can be found under **Admin > Settings > Custom project templates**.
+
+Within this section, you can configure the group where all the custom project
+templates are sourced. Every project directly under the group namespace will be
+available to the user if they have access to them. For example, every public
+project in the group will be available to every logged user. However,
+private projects will be available only if the user has view [permissions](../permissions.md)
+in the project:
+
+- Project Owner, Maintainer, Developer, Reporter or Guest
+- Is a member of the Group: Owner, Maintainer, Developer, Reporter or Guest
+
+Projects below subgroups of the template group are **not** supported.
+
+Repository and database information that are copied over to each new project are
+identical to the data exported with [GitLab's Project Import/Export](../project/settings/import_export.md).
+
+If you would like to set project templates at a group level, please see [Custom group-level project templates](../group/custom_project_templates.md).
\ No newline at end of file

doc/user/group/custom_project_templates.md

+# Custom group-level project templates **[PREMIUM ONLY]**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/6861) in [GitLab Premium](https://about.gitlab.com/pricing) 11.6.
+
+When you create a new project, creating it based on custom project templates is
+a convenient option to bootstrap from an existing project boilerplate.
+The group-level setting to configure a GitLab group that serves as template
+source can be found under **Group > Settings > General > Custom project templates**.
+
+Within this section, you can configure the group where all the custom project
+templates are sourced. Every project directly under the group namespace will be
+available to the user if they have access to them. For example, every public
+project in the group will be available to every logged in user. However,
+private projects will be available only if the user has view [permissions](../permissions.md)
+in the project. That is, users with Owner, Maintainer, Developer, Reporter or Guest roles for projects,
+or for groups to which the project belongs.
+
+Projects below subgroups of the template group are **not** supported.
+
+Repository and database information that are copied over to each new project are
+identical to the data exported with [GitLab's Project Import/Export](../project/settings/import_export.md).
+
+If you would like to set project templates at an instance level, please see [Custom instance-level project templates](../admin_area/custom_project_templates.md).
\ No newline at end of file