Relabel all code related to "Markdown" category

Gemfile

doc/api/markdown.md

doc/user/markdown.md

ee/spec/features/markdown/markdown_spec.rb

lib/api/markdown.rb

spec/benchmarks/banzai_benchmark.rb

spec/features/markdown/copy_as_gfm_spec.rb

spec/features/markdown/gitlab_flavored_markdown_spec.rb

spec/features/markdown/json_table_spec.rb

spec/features/markdown/kroki_spec.rb

spec/features/markdown/markdown_spec.rb

spec/features/markdown/math_spec.rb

spec/features/markdown/sandboxed_mermaid_spec.rb

spec/lib/banzai/color_parser_spec.rb

spec/lib/banzai/cross_project_reference_spec.rb

spec/lib/banzai/filter/absolute_link_filter_spec.rb

spec/lib/banzai/filter/asset_proxy_filter_spec.rb

spec/lib/banzai/filter/attributes_filter_spec.rb

spec/lib/banzai/filter/audio_link_filter_spec.rb

spec/lib/banzai/filter/autolink_filter_spec.rb