arduino-ide-extension README.md being outdated

[InstantMuffin]

Describe the problem

Trying to fix #1291 and to get a more general understanding of how the IDE works behind the scenes to maybe help out in the future, I found that README.md in the arduino-ide-extension folder seems to be quite outdated because it has a couple of dead links.

To reproduce

Click on a number of links in that document, for example the ones referencing the serial monitor are broken/not up to date.

Expected behavior

Links that work.
(I'm sorry I have to fill out this field.)

Arduino IDE version

"All of them"

Operating system

N/A

Operating system version

Additional context

No response

Issue checklist

• I searched for previous reports in the issue tracker
• I verified the problem still occurs when using the latest nightly build
• My report contains all necessary details

[per1234]

Thanks @InstantMuffin!

My recommended solution is to set up an automated link check system using the excellent markdown-link-check tool, as is the standard in Arduino tooling projects.

We have a standardized "template" GitHub Actions workflow for that purpose here:

https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/check-markdown-task.md

There is an alternative version for projects where it is not considered desirable to use the Task task runner (which is the standard for our Go-based projects):

https://github.com/per1234/.github/blob/main/workflow-templates/check-markdown.md