Configure a markdown linter for editing our lesson source files

[bielsnohr]

This was mentioned in #437 which I think was prematurely closed. The formatting / linting of markdown files across our lesson contain many inconsistencies, and often leads to me getting sidetracked fixing things that should be enforced by a linter/formatter. markdownlint seems like the tool for the job, so I will draft up a config file that enforces The Carpentries Workbench Style and makes editing easier for us.

[bielsnohr]

Having started to go at this, I am realising that we will probably need a mix of linter and formatter. The case that is proving difficult to solve:

1. Find the SSH URL of the software project repository to clone from your GitHub account.
  Make sure you do not clone the original repository but rather your own fork,
  as you should be able to push commits to it later on.
  Also make sure you select the **SSH** tab and not the **HTTPS** one.
  For this course, SSH is the preferred way of authenticating when sending your changes back to GitHub.
  If you have only authenticated through HTTPS in the past,
  please follow the [guidance to add an SSH key to your GitHub account](./installation-instructions.html#secure-access-to-github-using-git-from-command-line).

  ![](fig/clone-repository.png){alt='URL to clone the repository in GitHub' .image-with-shadow width="800px" }

2. Make sure you are located in your home directory in the command line with:
  
  ```bash
  $ cd ~
  ```

3. From your home directory in the command line, do:
  
  ```bash
  $ git clone git@github.com:<YOUR_GITHUB_USERNAME>/python-intermediate-inflammation.git
  ```
  
  Make sure you are cloning your fork of the software project and not the original repository.

The problem is indentation with multi-line ordered list items. Without the 3 spaces (it currently has 2), the sentence in item 3 after the code block will not get included in that list item and instead will be treated as a new paragraph, making the rendering look incorrect.

I think the solution is to use something like Prettier to be quite strict about this, but I have run out of time to look at that presently.