Add custom markdownlint rules to Operator, Runner, Charts and Omnibus projects

Relevant thread: gitlab-com/localization/tech-docs-forked-projects/prod/gitlab-operator!32 (comment 2864389601)

Localization team implemented custom markdownlint rules for emphasis formatting (to catch formatting issues in Japanese translations) in the upstream GitLab project: Docs formatting checks using markdown linting (gitlab-org/gitlab!209414 - merged) , and now we need to add the same validation rules to 4 other projects with docs

Tracking changes to all the remaining projects:

Project	MR	Current Status	Maintainer Review	Merged
Omnibus	Docs formatting checks using markdown linting - Omnibus	Pipelines passed - pending review
Operator	Docs formatting checks using markdown linting - Operator	Pipelines passed - pending review
Runner	Docs formatting checks using markdown linting - Runner	Pipelines passed - pending review
Charts	Docs formatting checks using markdown linting - Charts	Pipelines passed - pending review