Create section for @custom component templates

Currently, `@custom` component templates are explained only in this [Apply the polic](https://www.elastic.co/docs/manage-data/lifecycle/index-lifecycle-management/tutorial-customize-built-in-policies#example-using-index-lifecycle-policy-apply-policy)y section of the ILM docs. I think we should move it into the [Templates](https://www.elastic.co/docs/manage-data/data-store/templates) page.

As well, on the "Enable logsdb for integrations" page being added in [this PR](Enable logsdb for integrations), the first line could be updated to link specifically to the section about custom component templates.