Docs: describe supported custom issue trackers

Improve Custom Issue Tracker docs to better describe
supported issue trackers

doc/user/project/integrations/custom_issue_tracker.md

-# Custom Issue Tracker Service
+# Custom Issue Tracker service
 
-To enable the Custom Issue Tracker integration in a project, navigate to the
-[Integrations page](overview.md#accessing-integrations), click
-the **Customer Issue Tracker** service, and fill in the required details on the page as described
-in the table below. You will be able to edit the title and description later as well.
+To enable the Custom Issue Tracker integration in a project:
 
-| Field | Description |
-| ----- | ----------- |
-| `title`   | A title for the issue tracker (to differentiate between instances, for example). |
-| `description`   | A name for the issue tracker (to differentiate between instances, for example) |
-| `project_url`   | The URL to the project in the custom issue tracker. |
-| `issues_url`    | The URL to the issue in the issue tracker project that is linked to this GitLab project. Note that the `issues_url` requires `:id` in the URL. This ID is used by GitLab as a placeholder to replace the issue number. For example, `https://customissuetracker.com/project-name/:id`. |
-| `new_issue_url` | Currently unused. Will be changed in a future release. |
+1. Go to **{settings}** **Settings > Integrations**.
+1. Click **Custom Issue Tracker**
+1. Fill in the tracker's details, such as title, description, and URLs.
+   You will be able to edit these fields later as well.
 
-Once you have configured and enabled Custom Issue Tracker Service you'll see a link on the GitLab project pages that takes you to that custom issue tracker.
+   These are some of the required fields:
+
+   | Field           | Description |
+   | --------------- | ----------- |
+   | **Title**         | A title for the issue tracker (for example, to differentiate between instances). |
+   | **Description**   | A name for the issue tracker (for example, to differentiate between instances). |
+   | **Project URL**   | The URL to the project in the custom issue tracker. |
+   | **Issues URL**    | The URL to the issue in the issue tracker project that is linked to this GitLab project. Note that the `issues_url` requires `:id` in the URL. This ID is used by GitLab as a placeholder to replace the issue number. For example, `https://customissuetracker.com/project-name/:id`. |
+   | **New issue URL** | Currently unused. Will be changed in a future release. |
+
+1. Click **Test settings and save changes**.
+
+After you configure and enable the Custom Issue Tracker service, you'll see a link on the GitLab
+project pages that takes you to that custom issue tracker.
 
 ## Referencing issues
 
-- Issues are referenced with `ANYTHING-<ID>`, where `ANYTHING` can be any string in CAPS and `<ID>`
-is a number used in the target project of the custom integration (for example, `PROJECT-143`).
-- `ANYTHING` is a placeholder to differentiate against GitLab issues, which are referenced with `#<ID>`. You can use a project name or project key to replace it for example.
-- When building the hyperlink, the `ANYTHING` part is ignored, and links always point to the address
+Issues are referenced with `<ANYTHING>-<ID>` (for example, `PROJECT-143`), where `<ANYTHING>` can be any string in CAPS, and `<ID>`
+is a number used in the target project of the custom integration.
+
+`<ANYTHING>` is a placeholder to differentiate against GitLab issues, which are referenced with `#<ID>`. You can use a project name or project key to replace it for example.
+
+When building the hyperlink, the `<ANYTHING>` part is ignored, and links always point to the address
 specified in `issues_url`, so in the example above, `PROJECT-143` would refer to
 `https://customissuetracker.com/project-name/143`.