Fix markdown to render correctly

What does this MR do?

The documentation here: https://docs.gitlab.com/ce/integration/shibboleth.html doesn't render well.

This MR attempts to slightly restructure and fix a few problems in the Markdown so it renders properly.

Here's what it looks like now: http://review-docs-docs-ohe3pf.178.62.207.141.xip.io/ce/integration/shibboleth.html.

Related issues

It may or may not relate to: https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/108.

Author's checklist

• Apply the correct labels and milestone
• 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