If you mean while editing a project, I don't think this is possible. Also, I'm not sure it would make sense, since when one is in edit mode, one should have access to the files inside the project. So even if read-only was implemented, the user should still have the ability to toggle that and gain access to the file.
May I ask what is wrong with viewing the html/md page in a VLE/some other web server, Word/Google Docs, or a local md viewer? I actually think alt-tabbing to a different window for instructions is less confusing than viewing a read only document tab inside Construct, especially when there are lots of tabs open.