Add refined sample linting configuration

What does this MR do?

Having used proselint and markdownlint for a while now, I can offer some refined sample configuration.

While here, I thought I might make sure our section of the contributing guides adheres to the sample listing config and made some minor Markdown improvements.

Related issues

Very loosely related to: https://gitlab.com/gitlab-org/gitlab-ce/issues/49734.

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