Add markdown and package.json lint command

[t-hamano]

This PR adds lint commands for markdown files and package.json to improve code quality.

• Can use the new lint:md-docs or lint:pkg-json commands.
• Updated some markdown files to fix errors found when running lint:md-docs
• Markdown files are also linted when you commit.
• GitHub Actions that run when a PR is submitted will lint the .md and package.json files.

[mikachan]

Nice, LGTM. Thank you!