Have Markdown lint runs after commit

[bact]

As suggested in #723 (review)

This lint should be run for both of these repos

• Model specs: https://github.com/spdx/spdx-3-model/
• Other docs, including annexes: https://github.com/spdx/spdx-spec/

One of the possible tools is markdownlint:

• CLI: https://github.com/DavidAnson/markdownlint-cli2
• GitHub Action: https://github.com/marketplace/actions/markdownlint-cli2-action

Other relatedpre-commit hooks:

- repo: https://github.com/pre-commit/pre-commit-hooks
    hooks:
      - id: end-of-file-fixer
        stages: [commit]
      - id: mixed-line-ending
        stages: [commit]
      - id: trailing-whitespace
        args: [--markdown-linebreak-ext=md]
        stages: [commit]

Note the --markdown-linebreak-ext argument for trailing-whitespace hook. It is to preserved an intentional line break in Markdown -- according to the Markdown syntax: "When you do want to insert a <br /> break tag using Markdown, you end a line with two or more spaces, then type return." We can omit this argument if we are certain that in our spec there will be no such use / discourage such use.

[bact]

Some newline and whitespace rules can be enforce at the editor and checkout level - see #739