New test that checks markdown links for issues

[wildmanonline]

This PR introduces a test that checks all markdown links within our guides and determines if there are any issues. It also checks for duplicate aliases.

Here are the links that are ignored:

• Correctly formatted links to external sites

Here are the types of issues that are flagged by this test:

• Target not found (FAILURE): A link does not point to a guide or alias of a guide
• Duplicate aliases (FAILURE): The same alias has been added to multiple guides
• Slug mismatch (FAILURE): The guide's slug does not match the name of its parent folder
• Contains the linode docs domain (FAILURE): A link contains linode.com/docs
• Incorrect root (FAILURE): A link points to a guide but does not start with /docs
• Formatting issues (FAILURE): A link contains multiple slashes or does not start with a slash.
• Points to alias (WARNING): A link points to an alias of a guide. This is the only issue that does not result in a failure. This means we should run the update-internal-links.py script. python3 ./scripts/update-internal-links.py

Command to run script locally (when in the top level directory of your local repo):

python3 ./ci/check-links.py

Optionally add the --fix_issues argument to attempt to fix some issues. Currently the only issues it can fix are slug mismatches.

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[wildmanonline]

View the test results for the latest commit. There are several accurate failures shown that already exist in our library.