Add markdownlint CI job for docs syntax
This MR adds a markdownlint job to check the documentation source syntax.
Maintenance will be performed on git.ligo.org, containers.ligo.org, and docs.ligo.org on Tuesday 1 October 2024 starting at approximately 9am PDT. It is expected to take around 30 minutes and there will be several periods of downtime throughout the maintenance. Please address any comments, concerns, or questions to the helpdesk.
This MR adds a markdownlint job to check the documentation source syntax.