Fix links and Markdown.

What does this MR do?

Mainly fixes a broken link to a previous version of the documentation. This was invalid: https://gitlab.com/gitlab-org/gitlab-ce/blob/8-17-stable-ce/doc/administration/pages/index.md.

While here, moved a few URLs to be in-line, fixed some Markdown that might cause future linting issues and other minor improvements.

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