Add Web IDE docs

doc/user/project/repository/web-ide/index.md

+# Web IDE
+
+> Introduced in [GitLab Enterprise Edition Ultimate][ee] 10.4.
+
+> **Warning:** Web IDE is in **Beta**. For the latest updates, check the
+> associated [epic](https://gitlab.com/groups/gitlab-org/-/epics/26).
+
+The Web IDE editor makes it faster and easier to contribute changes to your
+projects by providing an advanced editor with commit staging.
+
+## Enable the Web IDE
+
+While in the early stages of the Beta, access to the Web IDE is by opting in.
+
+To enable the Web IDE, click on your profile image in the top right corner and
+navigate to `Settings → Preferences`, check `Enable Web IDE` and save.
+
+![Enable Web IDE](img/enable-web-ide.png)
+
+## Open the Web IDE
+
+Once enabled, the Web IDE can be opened when viewing a file, from the
+repository file list.
+
+![Open Web IDE](img/open-web-ide.png)
+
+## Commit changes
+
+Changed files are shown on the right in the commit panel. All changes are
+automatically staged. To commit your changes, add a commit message and click
+the 'Commit Button'.
+
+![Commit changes](img/commit-changes.png)
+
+[ee]: https://about.gitlab.com/gitlab-ee/