Updated Troubleshooting documentation: simpler mimetype workaround for .js file

[blep]

Description

Updated Troubleshooting documentation:

• Added an introduction advising users to check the developer console log for errors.
• Provided an example of an error message in the developer console log for an incorrect MIME type for a .js file.
• Provided a simple solution to fix the incorrect MIME type for a .js file that does not require an O.S. level change.
• Restructured the existing MIME type documentation to improve readability.

Fixes #2046 (issue)

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[tim-schilling]

I'm not a big fan of putting the python workaround before the more correct, fix your OS configuration, solution, but I can live with it. It suggests that's our recommended approach and if it is, we should change how the library works. I'm unsure if we want to do that, so that's why I'm not sold on the order.

That said, the rest of the changes are excellent, so I'm overall really happy to have this in.