Fix links in burndown charts page

What does this MR do?

Many links in this page were broken or weren't correct links at all.

Aspects of the page didn't conform to Styleguide standards.

Related issues

Relates to: https://gitlab.com/gitlab-org/gitlab-ce/issues/51696

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards