Markdown linting

What does this MR do?

Implements linting of Markdown files similarly to the GitLab documentation:

• markdownlint-cli2 for consistent Markdown style
• lychee for detecting broken links and URLs
• vale with GitLab style guideline rules for prose
• typos for detecting typing and spelling errors

The MR also introduces a Makefile with the following targets:

$ make help
TARGETS:
  lint-markdown-style             checks Markdown files for style and formatting issues
  lint-markdown-links             checks Markdown files for broken links
  lint-markdown-spelling          checks Markdown files for spelling and typing errors
  lint-markdown-prose             checks Markdown files for writing style issues.
  lint-markdown                   performs all linting tasks for Markdown files
  fix-markdown-style              resolves fixable style and formatting issues in Markdown files
  fix-markdown-spelling           resolves spelling and typing errors in Markdown files
  fix-markdown                    resolves all fixable issues in Markdown files
  install-tools                   installs or upgrades linting tools (Homebrew required)
  sync-vale                       download latest Vale styles
  help                            prints this message