Track subheading length improvements

I'll use this issue to track the process of cleaning up subheadings in the hope of getting a subheading-length rule in place.

Marcel wrote out the markdownlint rule:

  line-length:                # MD013
    code_blocks: false
    tables: false
    headings: true
    heading_line_length: 100
    line_length: 1000

• To run markdownlint against all files, markdownlint-cli2 "**/*.md"
• To specify the config file: markdownlint-cli2 --config .markdownlint-cli2.yaml 'doc/**/*.md'