Add limitations to SSE doc

- Add limitations section
- Link from project's index

doc/user/project/index.md

@@ -103,6 +103,7 @@ When you create a project in GitLab, you'll have access to a large number of
 - [License Compliance](../compliance/license_compliance/index.md): approve and blacklist licenses for projects. **(ULTIMATE)**
 - [Dependency List](../application_security/dependency_list/index.md): view project dependencies. **(ULTIMATE)**
 - [Requirements](requirements/index.md): Requirements allow you to create criteria to check your products against. **(ULTIMATE)**
+- [Static Site Editor](static_site_editor/index.md): quickly edit content on static websites without prior knowledge of the codebase or Git commands.
 
 ### Project integrations
 

doc/user/project/static_site_editor/index.md

@@ -82,3 +82,8 @@ company and a new feature has been added to the company product.
 1. The file is opened in the Static Site Editor.
 1. You edit the file right there and click **Submit changes**.
 1. A new merge request is automatically created and you assign it to your colleague for review.
+
+## Limitations
+
+- Currently, the Static Site Editor only works for files ending in `.md`. For example, it will not work for a file `index.html.md.erb` while it works for `index.html.md`.
+- The Static Site Editor still cannot be quickly added to existing Middleman sites. Follow this [epic](https://gitlab.com/groups/gitlab-org/-/epics/2784) for updates.