Fix links in burndown charts page

- Some link were broken.
- Some links were non-existant.
- Made burndown charts page adhere to style guide.
- Other minor improvements.

doc/user/project/milestones/burndown_charts.md

 # Burndown Charts **[STARTER]**
 
 > **Notes:**
-> - [Introduced][ee-1540] in [GitLab Starter 9.1][ee-9.1] for project milestones.
-> - [Introduced][ee-5354] in [GitLab Premium 10.8][ee-10.8] for group milestones.
-> - [Added][ee-6495] to [GitLab Starter 11.2][ee-11.2] for group milestones. 
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1540) in [GitLab Starter](https://about.gitlab.com/pricing/) 9.1 for project milestones.
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5354) in [GitLab Premium](https://about.gitlab.com/pricing/) 10.8 for group milestones.
+> - [Added](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6495) to [GitLab Starter](https://about.gitlab.com/pricing/) 11.2 for group milestones.
 > - Closed or reopened issues prior to GitLab 9.1 won't have a `closed_at`
 >   value, so the burndown chart considers them as closed on the milestone
 >  `start_date`. In that case, a warning will be displayed.
@@ -24,7 +24,7 @@ For an overview, check the video demonstration on [Mapping Work Versus Time, Wit
 
 ## Use cases
 
-Burndown Charts, in gerenal, are used for tracking and analyzing the completion of
+Burndown Charts, in general, are used for tracking and analyzing the completion of
 a milestone. Therefore, their use cases are tied to the
 [use you are giving to your milestone](index.md#use-cases).
 
@@ -32,15 +32,15 @@ To exemplify, suppose you lead a team of developers in a large company,
 and you follow this workflow:
 
 - Your company set the goal for the quarter to deliver 10 new features for your app
-in the upcoming major release
+  in the upcoming major release.
 - You create a milestone, and remind your team to assign that milestone to every new issue
-and merge request that's part of the launch of your app
+  and merge request that's part of the launch of your app.
 - Every week, you open the milestone, visualize the progress, identify the gaps,
-and help your team to get their work done
+  and help your team to get their work done.
 - Every month, you check in with your supervisor, and show the progress of that milestone
-from the Burndown Chart
+  from the Burndown Chart.
 - By the end of the quarter, your team successfully delivered 100% of that milestone, as
-it was taken care of closely throughout the whole quarter
+  it was taken care of closely throughout the whole quarter.
 
 ## How it works
 
@@ -51,8 +51,8 @@ Find your project's **Burndown Chart** under **Project > Issues > Milestones**,
 and select a milestone from your current ones, while for group's, access the **Groups** dashboard,
 select a group, and go through **Issues > Milestones** on the sidebar.
 
-> **Note:** You're able to [promote project][promote-milestone] to group milestones and still
-> see the **Burndown Chart** for them, respecting license limitations.
+NOTE: **Note:**
+You're able to [promote project](https://docs.gitlab.com/ee/user/project/milestones/#promoting-project-milestones-to-group-milestones) to group milestones and still see the **Burndown Chart** for them, respecting license limitations.
 
 The chart indicates the project's progress throughout that milestone (for issues assigned to it).
 
@@ -63,10 +63,10 @@ Since it only tracks when an issue was last closed (and not its full history), t
 assumes that issue was open on days prior to that date. Reopened issues are
 considered as open on one day after they were closed.
 
-Note that with this design, if you create a new issue in the middle of the milestone period 
-(and assign the milestone to the issue), the Burndown Chart will appear as if the 
-issue was already open at the beginning of the milestone. A workaround is to simply 
-close the issue (so that a closed timestamp is stored in the system), and reopen 
+Note that with this design, if you create a new issue in the middle of the milestone period
+(and assign the milestone to the issue), the Burndown Chart will appear as if the
+issue was already open at the beginning of the milestone. A workaround is to simply
+close the issue (so that a closed timestamp is stored in the system), and reopen
 it to get the desired effect, with a rise in the chart appearing on the day after.
 This is what appears in the example below.
 
@@ -74,9 +74,3 @@ The Burndown Chart can also be toggled to display the cumulative open issue
 weight for a given day. When using this feature, make sure issue weights have
 been properly assigned, since an open issue with no weight adds zero to the
 cumulative value.
-
-[ee-1540]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1540
-[ee-9.1]: https://about.gitlab.com/2017/04/22/gitlab-9-1-released/#burndown-charts-ees-eep
-[ee-5354]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5354
-[ee-10.8]: https://about.gitlab.com/2017/04/22/gitlab-10-8-released/#burndown-charts-eep-eeu
-[promote-milestone]: https://docs.gitlab.com/ee/user/project/milestones/#promoting-project-milestones-to-group-milestones