Remove exclusions from markdownlint

https://gitlab.com/gitlab-com/content-sites/handbook/-/blob/main/.markdownlint-cli2.jsonc currently excludes a number of folders

• 223 "content/handbook/business-technology/**/*.md", | !5804 (merged)
• 983 "content/handbook/customer-success/**/*.md", !6103 (merged)
• 528 "content/handbook/engineering/**/*.md", | !5929 (merged)
• 7736 "content/handbook/marketing/**/*.md", | !6100 (merged)
• 234 "content/handbook/product/**/*.md", | !5892 (merged)
• 2269 "content/handbook/sales/**/*.md", | !6102 (merged)