Contributing: Building docs locally

[StackScribe]

Transcribe information from the "Building the documentation locally" and "Building markdown files without Hugo" sections of the docs/CONTRIBUTING.md file.

Include a link in the docs/CONTRIBUTING.md file to this material.

Coordinate with the owner of #999 to determine how to handle info about the markdownlint tool. A couple specific items to be sure are included somewhere:

• The markdownlint README file lists the rules that are checked by the tool, with links to supporting documentation that gives details of the style that is being enforced.
• Many (most?) of the errors identified by markdownlint can be fixed automatically by running the following command from the directory where you run Hugo locally:

  make markdownlint-fix
  

  You must then git add, commit, and push the changes made by markdownlintto clear the CI build errors for your PR.